add optional anchor time to runs [#186531175]

[uraniumanchor]

Contributing to the Donation Tracker

• I've added tests or modified existing tests for the change.
• I've humanly end-to-end tested the change by running an instance of the tracker.

Issue from Pivotal Tracker

https://www.pivotaltracker.com/story/show/186531175

Description of the Change

This set of changes allows you to set a fixed start time for a given run. It will adjust the setup_time for the immediately preceding run to make sure that the start time is always where you set, or if it is not possible to do so, model validation will fail and whatever change you're trying to perform will not actually occur.

There's several touchpoints where this validation needs to kick in, for example moving a run required some changes because of new validation requirements. While looking for edge cases I discovered one that I'm not sure can easily be reconciled, so for now, you will get an error if you try and move a run directly before an anchored run (because the flex block is suddenly in a different place and I couldn't come up with a good answer for what to do with the old flex block). If you find yourself needing to do this for some reason, the only solution is to remove the anchor time before performing the move. It's possible I'll make this a more explicit error case in the future, but for now it will just fail because the current logic cannot come up with a solution, so the move is prevented.

As part of this, I had to touch up the current schedule editor to display errors that could never occur before, but it was relatively simple to do so.

Verification Process

Attempting to edit runs by hand in the admin, or via the schedule editor, ensuring that flex blocks would either error or end up with the expected values.

[faultyserver]

lgtm overall. nothing stands out. glad to have the ms fields on runs now, too lol