Create separate CSS / LESS files

[slhck]

We could split up bootstrap_and_overrides.css.less.erb into multiple .less files. One would just have to:

• Convert application.css to application.css.less
• Delete all the Sprockets directives
• @import each individual file in the directory from application.css. Do this via require_self and individual requires.
• Remove the .erb extension from any files that have it, and change ERB asset helpers to less-rails asset helpers.
• Make sure annotations.css.less is imported after bootstrap_and_overrides – this is why it's usually not a good idea to use require_tree ., since you can't control the order in which the files are loaded. The way you have it now, annotations.css.less would be loaded before bootstrap_and_overrides – before the variable you want to use even exists.

We can use asset-path LESS helpers from the less-rails gem, which is already included.