Applications currently support !action commands, like !claim, in email replies.

Now that all applications are built on transactions, we should expand the power of these actions to make them easier to use, more flexible, and support more specialized use cases.

In particular, we should make these changes:

• Support multiple !action commands per email.
• Support commands in creation email, not just reply email.
• Parse !action commands anywhere in email bodies (probably? Or anywhere at the beginning or end of an email body, just not in the middle? Not sure about how many false positives we'll hit).
• Add more available !action commands.
• Regularize documentation of !action commands so we don't have to enumerate them all in every email.

See some discussion in T6819. I have about half a diff for this, but it's likely very out of date. Desired result is for something like this to work:

!assign epriestley
!subscribe #whatever
!projects #urgent #red-colored
!status broken
!priority urgent

System xyz is literally on fire.