markdown: don't auto-mailto:-link a@b in @a@b mastodon UID

This chases a similar report for github posted at
  https://101010.pl/@mcc@mastodon.social/110742090990556162
in that "@nabijaczleweli@101010.pl" is autolinked as
        "@<mailto:nabijaczleweli@101010.pl>"
which sucks for obvious reasons
(it's not actually a correct mail address,
 the actual purpose is muddied by the autolinking).

Instead, match against "@?<previous mail regex>", and eject addresses
that start with @s as raw text.

srht/markdown.py

@@ -12,7 +12,7 @@ import mistletoe as m
 from mistletoe.span_token import SpanToken, RawText
 import re
 
-SRHT_MARKDOWN_VERSION = 15
+SRHT_MARKDOWN_VERSION = 16
 
 class PlainLink(SpanToken):
     """
@@ -24,7 +24,7 @@ class PlainLink(SpanToken):
     """
     pattern = re.compile(r"(?<!\\)(?:\\\\)*" # Fail if prefixed by odd number of backslashes
             r"((?P<url>[A-Za-z][A-Za-z0-9+.-]{1,31}://[^ \t\n\r\f\v<>]*)" # URLs: 'scheme'://'path'
-            r"|(?P<mail>[A-Za-z0-9.!#$%&'*+/=?^_`{|}~-]+@[A-Za-z0-9]" # Emails: 'user'@'domain
+            r"|(?P<mail>@?[A-Za-z0-9.!#$%&'*+/=?^_`{|}~-]+@[A-Za-z0-9]" # Emails: 'user'@'domain
                 r"(?:[A-Za-z0-9-]{0,61}[A-Za-z0-9])?"
                 r"(?:\.[A-Za-z0-9](?:[A-Za-z0-9-]{0,61}[A-Za-z0-9])?)+))")
     parse_inner = False
@@ -84,6 +84,8 @@ class SrhtRenderer(m.HTMLRenderer):
     def render_plain_link(self, token):
         template = '<a href="{target}">{inner}</a>'
         if token.mailto:
+            if token.target.startswith('@'): # mastodon "@user@domain" as plain text
+                return token.target
             target = 'mailto:{}'.format(token.target)
         else:
             target = self.escape_url(token.target)