Bring back context menu on Following feeds

[katagaki]

Previously removed due to weird sheet bug.
The sheet bug causes state in the sheet to get reset every time focus is taken away from the app (Lock Screen brought down, switched to App Switcher, Home Screen, etc).

Sheet bug still exists, but only for sheets opened from the Following tab. Currently being worked around with some SceneStorage tricks.

This bug needs to be fixed first before bringing back the context menu.