Mathvariant fixes

[brucemiller]

First step, clean up erroneous, missing and/or redundant font declarations which lead to erroneous or redundant mathvariant declarations in the MathML. Typically happens when the appropriate font is not propagated while synthesizing symbols.

Then add classes, like ltx_mathvariant_italic, for cases where the mathvariant cannot be handled by remapping the Unicode.

Fixes #2021 Fixes #2051

[brucemiller]

Following offline discussions, simplified the m:mtext (gets CSS, but not mathvariant), and avoid visual styling of formatting codepoints such as invisible-times. @dginev please take another peak if all looks OK.