content: Highlight non-silent mentions of the self-user

[chrisbobbe]

This probably makes sense to do after #646.

We should highlight non-silent mentions of the self-user, following web, so that users can easily see when a mention is intended to get their attention.

For some examples (and here I'm logged in as the "Chris Bobbe" account):

Notice the highlighting is done in two ways:

• The text of the mention itself is styled
• The whole message gets an interesting background color

[chrisbobbe]

(Should we add this to #309?)

[gnprice]

Thanks. Yeah, like #646, this is part of #157.

I think rather than add to #309, we can just rely on the a-content label and the milestones.

[alya]

In case it's helpful, note the unsubscribed channel edge case: https://github.com/zulip/zulip/pull/27765.

[chrisbobbe]

The web app also colors the background of the message's corresponding date separator (if any):

We should probably follow web.