change mute to unsubscribe to align with Github's own wording

Heya @velias! First off big thanks for the app, and for open sourcing it! :)

Here’s one thing which bugged me for a long time: »Mute Thread« is used nowhere on Github (at least not anymore). What it seems to do now is unsubscribe from the thread until the next mention comes. So I adjusted the wording to use »Unsubscribe«.