Backport #39337 by @nschloe
Follow-up to https://github.com/go-gitea/gitea/pull/38034.
bluemonday only keeps an element without attributes if it was registered
via `AllowNoAttrs`. The MathML rules only used
`AllowAttrs(...).OnElements(...)`, so plain `<mi>x</mi>`, `<mrow>`,
`<msqrt>` and friends were stripped to bare text. Since real-world
MathML is almost entirely attribute-less elements, nearly every formula
collapsed. The existing test missed it because its only case gave every
element an attribute.
Also fills the remaining gaps against MathML Core
(https://www.w3.org/TR/mathml-core/): `rowspan` on `mtd` alongside
`columnspan`, the `maction` element with `actiontype` and `selection`,
and the reserved global attributes `intent` and `arg`.
Tests cover attribute-less round-trips, table cell spans, and `maction`
with `intent`/`arg`.
Co-authored-by: Nico Schlömer <nschloe@users.noreply.github.com>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>
