Tidy dashboard editing

This, coupled with stagecraft changes that have already been merged, comprises an edit dashboard form and post action that should work.

Thoughts:

- We have no version control on dashboard changes any more - if we break something it may be difficult to get back.
- It's more difficult to dev against stagecraft dashboards locally than configs. Especially because we don't yet have everything in the form. Ben has suggested dev might still use the config files by default and that we create an easy task to post these configs to production once they are developed.
- We need smoke tests for this stuff - It's hard to be confident that changes we have made do no break create or other things. We are as confident as we can be in this case but we need a bug bash and we need to protect against regressions in future.