This change adds the ability for Review Bot to run tools manually. This

is used in two ways:

Tool configurations can now be set to only be run manually. In this

  case, the tool will create a status update in the NOT_YET_RUN state.
All tool configurations will mark the status update as retryable,

  which will allow the trigger of a retry in the case of error or

  timeout states.

Based on work by Alex Klemenchuk at /r/10316

Created a configuration set to only run manually. Verified that the

  initial status update was correctly created in the NOT_YET_RUN state,

  and that clicking the "Run" button triggered a tool run and subsequent

  update.
Triggered the run of a tool not normally set to run manually but

  without running the worker process (resulting in a timeout). Saw that

  the "Retry" button was shown and that clicking it re-ran the tool.
Tested that the correct diffset was chosen when running and retrying

  tools.