The usage update should really be a value...

- I was really bad at building this initially, so this is a fix of some
  sort by formalizing what was actually used (the manager use `1` to
  denote error, so changing the documentation to reflect that).
- Also provide these constants in the share models module.
Have the overview provide last API usage

- Keys are filtered out because we don't have an indirect primary key to
  limit credentials leakage.  So don't report that.
Fix sorting to use keys.
Use the shared helper method to format timestamps.
Have the Jabber frontend also report usage.