Persistent Dashboard refresh settings.

[alanmcruickshank]

Revision of #718
Also resolves #686

Dashboard refresh settings persist and option to set whether to use cache or not when fetching fresh data.

Required a fix of migration 27ae655e4247 to avoid problems with referencing models which weren't up to date yet. Have moved to declarative base.

New pull request because the history of the previous one was getting really tangled.

[coveralls]

Coverage increased (+0.02%) to 81.035% when pulling 6d9c45f2975f5e0daea2c9ceb8bd01e2aa4fd695 on alanmcruickshank:autorefresh-dashboard-4 into 2425b8f on airbnb:master.

[alanmcruickshank]

@mistercrunch I've rebased the changes up to the most recent master. I've noticed you've not merged this change in before, have you got questions/concerns about it? I'd love to get this into the next release, so let me know what you'd need to see for us to be able to do that.

[coveralls]

Coverage increased (+0.02%) to 81.516% when pulling c912643 on alanmcruickshank:autorefresh-dashboard-4 into aaef338 on airbnb:master.

[mistercrunch]

Sorry this never made it in, I'm cleaning up older PRs with merge conflicts at the moment. Please resubmit the PR if you want this feature to make it in.

[dmigo]

@alanmcruickshank was this one working? What was the impediment that prevented it from being merged?

[alanmcruickshank]

@dmigo It was working - but that was before at least one refactor of the dashboard (possibly many). I reckon some of this might still be salvageable but it would need some work.