Highlight or provide a TL;DR for unread messages

**Is your feature request related to a problem? Please describe.**

In group chats, especially ones with a relatively short-term purpose, sometimes there are situations where someone is busy and can't read the chat but 2 or more people are actively talking.

When this happens, there can be 10s to 100s of messages which often is a bit much for someone to read at once, so it's easier to just skip to bottom and catch-up. In most cases, missing the few important messages that might've been there.

Important messages enough that everyone should probably read, but not important enough that it needs to be pinned.

**Describe the solution you'd like**

I would like to be able to mark messages as "highlighted" or "important".

If a message has this attribute, then when a user comes online and has lots of unread messages, next to the "Scroll to Bottom" button there should be a TL;DR button.

The TL;DR button would open a modal with only the highlighted messages, so they can read the important ones that may be relevant to them.

**Describe alternatives you've considered**

A viable alternative could be if there was a menu somewhere similar to Discord or Slack that just display all messages from all chat rooms where you've been mentioned.
Then any important information can be mentioned to the user, and it'd appear in that list. The button to view mentions would display a badge when there are unread mentions.

However, one reservation I have about this is that a mention triggers a notification for most users. Often, conversations that are taken place like this don't require notifying the user, but just making sure it's convenient for them to access when they're back online.

In general, I would consider a TL;DR more practical though since it's local to a chat room and period of time.

**Additional context**

In my use-case, we make group chats for notable things. Perhaps a group vacation, or some other significant event. During this, there's usually lots of very helpful information being shared at random points in the conversation that ideally everyone should see, but not everyone is online at the time and hundreds of messages get sent between. At this point, people end up missing the important bits of the conversation.

The issue does not name files, tests, or an entry point. Start by reviewing the relevant Element client chat and unread-message flows, then clarify whether highlighted messages, a room-local TL;DR modal, or unread mentions are in scope. Done should include an agreed interaction design and acceptance criteria for surfacing important unread messages.

Written by the indexing model from the issue text.