improveProton per A2N

Remove 'get proton family' button and improve the placement of the mail label

--- main feature
Remove 'get proton family' button
Move the label just to the right of the sender name
A setting page where you can choose to use one option, both or nothing at all

--- history
I've been wanting to make a firefox extension for a long time and now I have two excuses for it:
- I pay proton and I'm not a fan of the "get a new plans" button everywhere because of it.
- As I use the label to sort my mail, I find it more readable to have the label just to the right of the sender's name.

--- Installation
Download the extension and check the options (by default both of them are deactivated)