MDL-45076 theme_bootstrapbase: Select placement in participants page

Usually the group selector is aligned with the label, but as we want
it to be consistent with the rest of the selects in the participants
page we need an override.

Conflicts:

	theme/bootstrapbase/style/moodle.css
MDL-45076 theme_bootstrapbase: Clean up participants controls LESS

Conflicts:

	theme/bootstrapbase/style/moodle.css