Move mathml.css to libs

Following @darkwing suggestion:

> move mathml.css to media/css/libs, because it's from an external lib.
Replace UA string test with mspace element test

Detect MathML by checking the support for the mspace element is more
reliable than using UA string. Thanks to @fred-wang and @darkwing.
Add notification

Add text before title warning user about use of web browser that
not support MathML.

TODO:

- Improve notification text
- Improve notification css
- l10n/i18n
Fix for bug at 3bf5149 and dc3bfed