Set a default background color on tracks

[danielgustafsson]

If the user doesn't specify a background color when configuring a track then the generated CSS is incorrect:

  .track-xyz {
    background-color: ;
    color: #000000;
  }

This adds a default color which is the inverse of the default text color to avoid incorrect CSS.

[mhagander]

Since it causes an issue in the templates we should also set blank=False on it I think?

And mind that changes to the model need to be back patched into the migrations. Since this doesn't actually make any database level changes you shouldn't create a new migration, but you should make modifications to the old one so django doesn't complain about them being missing.

[mhagander]

(also, please rebase on my fix of the tests, so it stops complaining about that)