FEATURE: encourage users to set chat thread titles

[dbattersby]

We want to encourage users to title their threads to make it easier for other users to join in on conversations that matter to them.

We want to account for the following:

• these changes apply only on mobile
• thread has at least 5 sent replies before showing prompt
• prompt should be timed to appear for 5 seconds
• progress bar showing how long it will remain visible
• can tap the Set title button to open a modal where they can add a thread title
• can tap the Don’t show again link to immediately dismiss this prompt
• can tap the X button to immediately close this prompt
• modal will include a promotion for Discourse AI
• don’t show prompt again on any other eligible thread (after clicking Don't show again)
• eligible users should only see this prompt once per thread (OP + admin)
• can tap the Thread header label to open the edit thread title modal (OP + admin)

[jjaffeux]

We probably need a controller spec too

[jjaffeux]

I think we can try to merge this Monday