Don't notify users without access to the project when they are (accidentally) mentioned in a note.

Addresses #2366.

See merge request !1216

Apply syntax highlighting to fenced code blocks client-side

Instead of applying the syntax highlighting scheme class to these blocks
server-side, we use Javascript and Gon to apply the user's color scheme
(or the default) client-side.

This will make it easier to cache these blocks in the future because
they're no longer state-dependent.

See merge request !1203

Update PreferencesHelper specs

Instead of rendering this value server-side, we use Javascript and Gon
to apply the user's color scheme (or the default) to any syntax
highlighted code blocks.

This will make it easier to cache these blocks in the future because
they're no longer state-dependent.

Very similar to Gitlab::Theme, this contains all of the definitions for
our syntax highlighting schemes.

Re-enable the "links with adjacent text" UserReferenceFilter spec

See merge request !1194