process: strip message IDs of whitespace

See discussion on sr.ht-discuss: per RFC 5322, whitespace is permitted
in these headers and needs to be stripped before interpreting the
Message-ID. This was breaking threading for some MUAs.
Improve accessibility of patch review pages
listssrht-lmtp: fix fallback address for . lists
patches.py: fix generated cover letter locals
Upgrade to Alpine 3.11
Improve config.example.ini
Adds pygit2 as a dependency in setup.py

pygit2 was being used in /listssrht/types/email.py, but it was never
declared as a dependency of the module.
Generate Next -> links with more patch context

Fixes #134
Improve patchset feedback rendering

To be honest, this code is a mess and will have to be completely
overhauled at some point.
Updates for single-sign-on support
Don't let lists.sr.ht email itself
listssrht-lmtp: more logging
Filter "normal" access from settings-access.html
Print envelope of rejected messages
Set server_default access to normal
Implement ListAccess.moderate
filters.py: Fix issue with patch detection
filters.py: format_body: parse patch
Fix 500 on incorrectly labelled patch threads

This often happens with hg patchsets which are not yet understood by