Markdown: Sanitizier Configuration (#9075)

* Support custom sanitization policy

Allowing the gitea administrator to configure sanitization policy allows
them to couple external renders and custom templates to support more
markup. In particular, the `pandoc` renderer allows generating KaTeX
annotations, wrapping them in `<span>` elements with class `math` and
either `inline` or `display` (depending on whether or not inline or
block mode was requested).

This iteration gives the administrator whitelisting powers; carefully
crafted regexes will thus let through only the desired attributes
necessary to support their custom markup.

Resolves: #9054

Signed-off-by: Alexander Scheel <alexander.m.scheel@gmail.com>

* Document new sanitization configuration

 - Adds basic documentation to app.ini.sample,
 - Adds an example to the Configuration Cheat Sheet, and
 - Adds extended information to External Renderers section.

Signed-off-by: Alexander Scheel <alexander.m.scheel@gmail.com>

* Drop extraneous length check in newMarkupSanitizer(...)

Signed-off-by: Alexander Scheel <alexander.m.scheel@gmail.com>

* Fix plural ELEMENT and ALLOW_ATTR in docs

These were left over from their initial names. Make them singular to
conform with the current expectations.

Signed-off-by: Alexander Scheel <alexander.m.scheel@gmail.com>

custom/conf/app.ini.sample

@@ -877,6 +877,12 @@ SHOW_FOOTER_VERSION = true
 ; Show template execution time in the footer
 SHOW_FOOTER_TEMPLATE_LOAD_TIME = true
 
+[markup.sanitizer]
+; The following keys can be used multiple times to define sanitation policy rules.
+;ELEMENT = span
+;ALLOW_ATTR = class
+;REGEXP = ^(info|warning|error)$
+
 [markup.asciidoc]
 ENABLED = false
 ; List of file extensions that should be rendered by an external command