Add a syntax highlight dropdown, if pygments is enabled.

Summary:

• Add a default list of supported languages to default.conf.php and make the initial/default value customizable.
• Store a '' in the database to infer the language from the filename/title.

Test Plan:
Tested in my sandbox with pygments enabled and disabled and various
combinations of filename/extension/dropdown selection.

Reviewers:
epriestley

CC:

Differential Revision: 587