admin search: use placeholder text & option-group

use the Bootstrap styles for an option-group to
connect the search button and text entry field.

Stops weird word-wrapping things with the button.

Not 100% sure on the accessability implications of
using placeholder instead of a label, could add a
sr-only label back too. Though the info I could
find suggested that adding more labels would confuse
browsers more, while just placeholder worked better.

Not compatible with IE8.
course search: add the form-control class

Needed to make the text entry look nice.

Similar comment about the use of placeholder to the last commit.
MDL42634: opacity causes trouble for dropdowns

Fix for a bug that's currently being worked on in core.

Opacity is a bit weird, so replace it with some lighten'd
colors using less color functions.
various tidy ups, add newlines, plus compile less
Merge pull request #144 from ds125v/blocks

Blocks