Make it possible to automatically log out inactive sessions after some time

[matrixbot]

This issue was originally created by @sandhose at matrix-org/matrix-authentication-service#1875.

Now that we're tracking session activity, it might be easier to do.
The idea is to be able to have sessions that automatically log out after some time.
This should not be done with all sessions, but rather have some filters applied. Not sure what the right semantic should be.

Once we figured out which session we want to automatically log out, implementation should be done as a background job running every now and then.