Use radio buttons only for math mode selection

While the combination of all redering modes, might
be useful for some power users it will confuse the
average user and makes bug reporting harder.

Therefore, source rendering on the server side is
used when MathJax is enabled on the client side.

Change-Id: I05a20d7c2298115dbf0429db6c107e8132cc15ff
Merge "Use radio buttons only for math mode selection"