Handle old experimental content features, or get them cleaned up

[gnprice]

The results of the survey #917 / #190 of unimplemented content features include several features that appear in the public chat.zulip.org message history for only a brief period. It's not clear any of them went to Zulip Cloud or into a published Zulip Server release.

They are (borrowing notes from @PIG208 on #921):

• <img>
  Oldest message: 1792633; newest message: 1792683 (3 messages)
  It was this experiment for markdown: Add support for ![alt text](url) syntax for images. zulip#29300.

• <span class="timestamp">
  Oldest message: 882554; newest message: 908075 (38 messages)
  Added -> Removed (example)

• <div class="message_embed">
  Oldest message: 192764; newest message: 193181 (28 messages)
  (example)

• <span class="inline-subscribe">
  Oldest message: 4297; newest message: 97826 (45 messages)
  Added -> Removed (example)

For each of these it'd probably be best to get them migrated on the server to a modern form.

Failing that, it'd be good to support them on the client.

When we go to pick up actually doing this, we can file individual issues for each of these features.

Chat thread: https://chat.zulip.org/#narrow/stream/243-mobile-team/topic/Unimplemented.20features/near/1932599