account activity log

I think that Matrix should have a standard way of exposing an account activity log. This would allow users to view, for example:

- when they logged in/logged out from different devices
- when they changed cross-signing keys
- when they created key backups
- when they changed passwords
- when they added/removed email addresses, phone numbers, ...
- etc.

This could be done as a room shared with a server user, similar to the system alerts room. This would also probably benefit from extensible events.

No file, test, or entry point is identified in the issue. Start by reviewing the Matrix specification's existing system-alert and extensible-event conventions, then determine how a standard account activity log would represent the listed account and device actions. Done means the proposal defines the event model and exposure mechanism clearly enough for the specification to adopt.

Written by the indexing model from the issue text.