src/httpserver.py: use HTML5.
CONTRIBUTING.md: bold the testing branch.
src/conf.py: fix gender-neutral pronouns.
Add invalid command was called in.

fixes #584
Merge pull request #697 from sckasturi/patch-2

Add the channel invalid command was called in.
Misc: Improve code in previous merge.
Merge pull request #695 from Mkaysi/patch-3

src/conf.py: fix gender-neutral pronouns.
Misc: Internationalize two strings.
Merge pull request #692 from Mkaysi/patch-1

CONTRIBUTING.md: bold the testing branch.
Misc: Only write channel name, instead of prepending it with “channel”.
update ChangeLog from git. Closes #700 .

[CI SKIP]
Merge pull request #699 from Mkaysi/changelog

Update ChangeLog from git.
Revert "update ChangeLog from git. Closes #700 ."

This reverts commit e88cbb8ba78212c0c4841899a02670294b8ca570.
Merge pull request #675 from Mkaysi/html5

src/httpserver.py: use HTML5.