Allow users to edit their own CSS and JS on Agora

Summary:
Not enabled by default, as it's a security risk, and such assessment
should be done wiki by wiki.

References:

• https://www.mediawiki.org/wiki/Manual:$wgAllowUserCss
• https://www.mediawiki.org/wiki/Manual:$wgAllowUserJs

Fixes T1889.

Test Plan: [[User:Dereckson/timeless.css]] should be loaded

Reviewers: dereckson

Reviewed By: dereckson

Maniphest Tasks: T1889

Differential Revision: https://devcentral.nasqueron.org/D3139