mathpreview: js-based math preview panel
- JQuery plugin to display math rendering (via MathJax) preview next to input fields or textareas.
- Plugged for submissions, and enabled via CFG_WEBSUBMIT_USE_MATHJAX. (closes #1221)
- Enriches Submission Guide in consequence.
- Updates MathJax configuration to enable '$' escaping, with tex2jax->processEscapes=true, as recommended in MathJax doc when using '$' as inline delimiters
Tested-by: Tibor Simko <tibor.simko@cern.ch>