MBS-6262, clean up release editor / form css.

1. move some inline styles to classes.
2. change some buttons to use the <button> element instead of <a>.
3. set box-sizing to border-box on form elements, so <input> and
   <select> have the same size.
4. group styles for each release editor tab together in release-editor.css.
Use button/menu context on buttons.
MBS-6262, Don't use a list when there's just a single value.
MBS-6262, form_row_select now accepts a 'class' argument instead of a generic 'attrs'.
MBS-6262, use less for some form element sizes.
MBS-6428, Move artist credit input styles to form.less, where it cat use existing variables.
Merge branch 'mbs-6262' into beta

* mbs-6262:
  MBS-6428, Move artist credit input styles to form.less, where it cat use existing variables.
  MBS-6262, use less for some form element sizes.
  MBS-6262, form_row_select now accepts a 'class' argument instead of a generic 'attrs'.
  MBS-6262, Don't use a list when there's just a single value.
  Use button/menu context on buttons.
  MBS-6262, clean up release editor / form css.