Add last_active column to users page

[alison985]

From upstream: getredash#789

[alison985]

@robotblake This may, in some respects double database writes upon each event in re:Dash. Specifically, when a recordable event call happens it will record the event to re:Dash's meta database and update the user's last_active_at time. Is that something you would have concerns about?

[rafrombrc]

Discussed in a meeting... we're not especially concerned about this, the write load is pretty low, let's try it out and see how things look in QA.

[rafrombrc]

With the caveat that we'd want some details on what, exactly, causes the last_active to be updated. We wouldn't want to write on every asset fetch, for instance. Is there already a user_id value in the events table, so that we could do a join to fetch this rather than writing to the user table?

[alison985]

@rafrombrc - The original concept from Arik was to update the user table. However, there is a user_id field in the events table that is a much better answer. Thanks!