Implement filter settings

Problem/Motivation

Now that MathJax is implemented as a text filter, it would be nice to use filter settings to allow the admin to control how MathJax runs on a given text format.

Proposed resolution

Implement filter settings if MathJax can be made to run that way.

Remaining tasks

• Investigate activating multiple MathJax configurations per page.
• Implement the appropriate settings callback.

User interface changes

If this change is made, site admins would now configure MathJax for each text format, with sensible defaults.

API changes

N/A