[WIP] - Mute topics from within ZT

[sumanthvrao]

This PR adds support to mute topics from within ZT on M keypress:

• The first commit here introduces API call to toggle muted status.
• The second adds a method in core which builds the text and calls the PopUpconfirmationView with the model function passed in as callback.
• We add functionality in buttons to restore/decrease unread count during appropriate events like (un)muting.
• We register the handler for muted topic event which handeles and event-based approach as well. This intern calls the mark_(un)muted methods of each buttons.
• We introduce the 'TOGGLE_MUTE_TOPIC' hotkey and maps it to its own category.
• Last commit involve connecting the Key added earlier on a topic button so that it will be able to perform action of muting/unmuting a topic.

[sumanthvrao]

Tests are yet to be added, but feedback is of-course welcome. @neiljp @amanagr

[zulipbot]

Heads up @sumanthvrao, we just merged some commits that conflict with the changes your made in this pull request! You can review this repository's recent commits to see where the conflicts occur. Please rebase your feature branch against the upstream/master branch and resolve your pull request's merge conflicts accordingly.