Distinguish muted users in UI

[sm-sayedi]

Note: No tests are included in this draft revision.

This PR aims to respect the user's settings for muting other users by distinguishing its related UI appearance. From the list of places in UI mentioned in https://zulip.com/help/mute-a-user, the following are included as they're compatible with the current scope of mobile:

• Combined feed sent by muted users, including the name, profile picture, and message content, are hidden behind a Click here to reveal banner. A revealed message can later be re-hidden.

Message from muted user (Before)	Message from muted user (After)
Revealed muted message	Revealed muted message (Long-pressed)

• Muted users have their name displayed as "Muted user" for emoji reactions, polls, and when displaying the recipients of group direct messages.

Emoji reactions | Poll (Before)	Emoji reactions | Poll (After)
Group DMs with muted user (Before)	Group DMs with muted user (After)
Recent DMs view (Before)	Recent DMs view (After)

• Recent conversations and other features that display avatars will show a generic user symbol in place of a muted user's profile picture.

Custom profile field (Before)	Custom profile field (After)

• Muted users are excluded from the autocomplete for composing a direct message or mentioning a user.

Fixes: #296