This fixes issue #27847 where regular expression allowed email address
to start with special symbols. Valid email addresses should start with
alphanumeric character, and as such will be rendered as email.
Added test cases from the bug report to validate, such input will not be
rendered anymore as email address.
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Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>
Remove unmaintainable sanitizer rules. No need to add special "class"
regexp rules anymore, use RenderInternal.SafeAttr instead, more details
(and examples) are in the tests
To make the markup package easier to maintain:
1. Split some go files into small files
2. Use a shared util.NopCloser, remove duplicate code
3. Remove unused functions
