Use toggle for subscribed/unsubscribed to notifications

Currently the only indicator that you are subscribed to issue is text in button like 'Subscribe' or 'Unsubscribe' (and some text below that no one reads).

This is not clearly visible. How we make it more visible?

Mockups

Expanded state uses toggle:

Screen_Shot_2018-01-08_at_9.37.17_AM

Screen_Shot_2018-01-08_at_9.55.25_AM

Collapsed state changes from rss icon to bell icon:

Screen_Shot_2016-11-28_at_12.33.32_PM

/cc @jschatz1 @JobV

Scope

Implement the toggle UI for both the issue page and the merge request page.

Edited by Taurie Davis