Hide "download add-on" link when user has add-on already installed

[miketaylr]

cf. webcompat/webcompat-reporter-extensions#34

This issue is about adding CSS to hide the relevant link -- the add-on will add a class to control this.

[miketaylr]

(there might be a dupe for this issue... I can't seem to find it)

[magsout]

Something like is-hidden looks good

[miketaylr]

Ah, cool -- that probably makes more sense to just add that directly to what we want to hide.

[magsout]

Ah it's the addon detects and adds the class

[miketaylr]

Yeah, but the addon can add the class to the download link directly.