This adds the two new resources tool_executions and tool_executables, and also

brings back the old review_bot_review resource (left unmodified).
At a high level, this API now supports multiple profiles/configurations per

tool, allows us to manually execute tools, and allows us to log and track the

status of tool executions.
A tool execution contains information on a tool profile that is currently

being executed or has been executed on some diff of a review request. This

resource supports getting a specified tool execution, getting a list of tool

executions for a review request ID and diff revision (with the optional

parameters status and get-latest, the latter of which will be very useful

for the manual execution UI), creating a new tool execution and adding an

execution request to the message queue, and updating a tool execution's status

and result (used by the workers).
The tool executables resource provides a list of tool profiles that the user is

allowed to manually execute, for some review request ID and diff revision.

Did GET, POST, PUT requests on tool-executions with all possible request

parameters, and GET requests on tool-executables.
Went through multiple manual execution scenarios (using the change in /r/6192).

- Got a list of executable tool profiles.

- Removed all manual permissions on a profile, and saw that it was no longer

  on the list of executables.

- Tried to create an execution, specifying a profile not on the list.

- Created an execution, specifying a profile on the list this time.

- Got the list of executions for the review request/diff, and saw the 'queued'

  status for the above execution.

- Saw that the execution request was received by a worker. The worker updated

  the tool execution with the 'succeeded' status and JSON review, and

  published a review on the review request. The last-updated time was also

  automatically updated.

- Checked the list of executable tool profiles, and saw that the just-executed

  profile was no longer on the list.

- Tried to execute the just-executed profile, and failed.


Tried passing invalid review request IDs, diff revisions, and profile IDs,

  and verified that the requests all failed.
Tried passing a profile ID for which the user did not have permission to

  manually execute, and verified that that the profile was not listed as an

  executable and a PERMISSION_DENIED error for the POST.
Tried to update the status of an execution (not logged in as the Review Bot

  user) and was denied permission.