Convert stylesheets to scss

So that we can use image-url
Fix path.

Seems clearer and more robust to specify it relative to rails root.
Add stylesheets to precompilation list
Fix image link.

Previously this pointed to a non-existent image. That's a problem with
the asset pipeline, so point it to what I assume was the intended
target.
Don't hide whole authority preview.

Just hide the list filter as the original main.css does. We want to use
this in select_authorities as well as select_authority, and want it to
be visible by default.
Add border to select.
Add styles for batch request authority selection.
Styles for list of authorities on request writing page.
Merge branch 'feature/batch-requests'

Conflicts:
    assets/stylesheets/custom.scss
    assets/stylesheets/main.css
    assets/stylesheets/main.css.scss
    assets/stylesheets/main.scss
    lib/alavetelitheme.rb