Fix "ONLY_FULL_GROUP_BY" issue in SystemAction queries

Summary: Ref T13404. This query is invalid under "sql_mode=ONLY_FULL_GROUP_BY". Rewrite it to avoid interacting with actorIdentity at all; this is a little more robust in the presence of weird data and not really more complicated.

Test Plan:

• Enabled "ONLY_FULL_GROUP_BY".
• Hit system actions (e.g., login).
  • Before: error.
  • After: clean login.
• Tried to login with a bad password many times in a row, got properly limited by the system action rate limiter.

Maniphest Tasks: T13404

Differential Revision: https://secure.phabricator.com/D20782