Disable-on-submit on submit buttons in the dashboard

The first commit changes the JS such that it tests for data-loading-text directly. This reduces API complexity at the cost of removing the possibility to get
the pure bootstrap behaviour that doesn't check for invalid fields. I
cannot imagine a use case where the pure bootstrap behaviour is better, so
I took the opportunity to reduce complexity.

The other commits should be self-explanatory.

I only tested a small random sample of the changes in the oscar dashboard.