Use lowercase letters and shorthand values for HEX colors where
possible.

Discussed in #14299.

Now we don't need to override the user color scheme on search results.
They'll look good with any of them.

Also preserve code preview color scheme in events dashboard.

Assign default colors to all code blocks shown as <pre class="code highlight [color_scheme]">

Closes #1139

I decided to create a fork of rouge as rouge lacks a HTML formatter with
the required options such as wrapping a line with <span> tags.
Furthermore I was not really convinced about the clarity of rouge's
source code.

Rugments 1.0.0beta3 for now only includes some basic linting and a new
HTML formatter. Everything else should behave the same.