read session_expire_delay from database if table & column exists;

Previous code only ends up using the default value for session_expire_delay, not the value stored in the database, making the session always 1 week.

See merge request !1217

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