Allows Status Updates to have tools that are manually run, and allows

Status Updates with a error/timeout state to be rerun.
Adds a new StatusUpdate state NEEDS_RUN which represents the ability

to be run manually on user interaction in a review request, and is

considered a pending StatusUpdate for the purposes of collapsing /

summaries.  
Adds a new option to the PUT API for Status Updates that supports

running a tool attached to a Status Update. This runs a new signal:

status_update_run_tool, which extensions can hook into to run some

sort of tool when the user manually hits the run/retry button in the

review request.
Adds a new button to the front-end to PUT to the new StatusUpdate API,

which checks for updates on newly run tools.
Adds a new field to the StatusUpdatesEntry model containing the review

request ID to be passed to a the event handler of these new buttons.

Tests have been added for all added functionality in front-end and

back-end.
All tests pass, and manual testing with reviewbot tools has been done

and works with the new manual run feature, as well as retrying features.