Following strings are marked translatable:
- All strings which are passed to `button.text` or which affect the
text of buttons.
- All strings passed to `placeholder`.
- All strings passed to `compose_error`.
Fixes#969
Add javascript to handle the button clicks and update the status based
on the subscribe and unsubscribe events from the server.
(imported from commit 6b9c0b40d9084e3d8b64bed701ebc786bef6d432)