Fix "An unexpected error has occurred" while vectorizing in recent versions of Firefox.

* Make Vectorizer concurrency customizable.
* Remove Retry checkbox from Vectorizer. We automatically do the right thing now.

Record prematurely pruned elements. This bumps the vector version number to 2, which means previous versions of the commandline tools will not understand these new vector files.

* Have the Evaluator skip over `about:` tabs that used to crash it with permission problems.

* When the Retry checkbox is checked in the Vectorizer, it now retries only errors it thinks are transient. This means you don’t have to wait 200 seconds for your syntax errors to bubble to the fore if you forget to uncheck Retry.
* Make vectorType optional if it’s the same as the trainee ID.

* Freezing pages now takes only 1/7 as long, thanks to an upgrade to the freeze-dry library.
* There's now a keyboard shortcut for freezing the current page: command-shift-O. This lets you capture hover states.
* The original URL of each frozen page is now preserved in an HTML attribute.
* Rather than always defaulting to 1024x768 for the Viewport Size in the Vectorizer, pull the default from the `viewportSize` of the selected trainee.
* Remove obsolete Trainer, pieces of which now remain solely as an Evaluator, for debugging rulesets.

* Add an Evaluate button to the Trainer, useful for ruleset debugging.
* Add a Base URL field to the Vectorizer, which saves a lot of find-and-replacing on page filenames.
* Add a Retry On Error checkbox to the Vectorizer so retrying can be disabled during ruleset debugging.
* Document Vectorizer.

* Switch to Fathom 3. This requires a fathom-trainees fork based on tag 3.0 or
later. Equivalently, you can make these changes:
* Switch to specifying coefficients as a Map instead of an Array. If you
don't want to name your rules, Fathom will auto-name them `_0`, `_1`, etc.,
in order of appearance in the ruleset.
* The `rulesetMaker` function no longer takes any params. In Fathom 3, all
weighting is done internal to the framework.
* All rules should return a value between 0 and 1, representing a confidence.
* Add the Vectorizer tool, which exports feature vectors for optimization with
Fathom 3's external tool.
* Fix additional causes of duplicate downloads when using the Corpus Collector
on recent versions of Firefox. I think they're really all gone now.

Fix Corpus Collector spewing extra bullet points, closing the wrong tabs, downloading duplicates, and generally misbehaving in recent Firefox versions (e.g. 65).

* Clicking a good/bad cell now takes you to that sample's tab and, in the case of a bad cell, indicates which element was wrongly selected, which is invaluable for debugging. Identifying the element requires a fathom-trainees fork that pulls in Fathom 2.8. If you have a custom success function, you must also add a third param to it and scribble a ``badElement`` property on the received object.
* Add a Pause button to the Trainer. This is useful while on battery power or to take the load off Firefox's JS thread so the dev tools can run quickly.

* Make viewport size customizable during capture.
* Bump default freeze delay from 0 to 1. This avoids many dead-object errors during bulk capture.
* Limit length of tags in devtools panel so they don't make it unusably wide.
* Focus the new label field after inspecting an element and switching to the Fathom devtools panel. This makes for faster entry.

Add computation of a 95% confidence interval for accuracy, using binomial proportion confidence intervals.

* Support customizable viewport sizes.
* Stop using `traineeCoeffs` message in favor of `trainee`, which is more future-proof. This requires a fathom-trainees fork based on d93bc593a08f6503d137df4d35ce2a2bc6b93b6e or later. Please merge in from fathom-trainees' latest master.

* Re-saving frozen pages should now be idempotent most of the time.
* Solve some causes of Corpus Collector freezing.
* Add progress reporting during freezing.
* Use the hostname as a download filename in the absence of a user-provided one.
* Use standard Photon menus for the toolbar button (since the stylesheets have now been documented).

Fix bug where some inspected elements would yield merely "false" in the Fathom dev panel.

Fix a packaging problem that was causing the dev panel not to work, due to a missing copy of the Simmer library.

Add a decent UI to the devtools panel. Now you can see what you've labeled!

Fix a bug where the Save button on the devtools panel wouldn't do anything unless an element was inspected.

* Add Trainer.
* Switch from npm to yarn.

* Fix the "No matching message handler" errors when downloading using the Corpus Collector.
* Switch from webpack to rollup.