#8355 OSHmail: add subheading

Add the text to the header image to ensure that it appears correctly in email clients
#8350 OSHmail handle YouTube videos

If a YouTube video is embedded in content in an OSHmail it will be
replaced with a link to the video on YouTube with a placeholder image
(taken from YouTube).
added protocol (https:) to the youtube URLs since several mail clients can't handle
the protocol-agnostic version. Refs #8350
remove 'is_default_page' from query inside sitemap - we don't seem to have that index in solr?? Fixes #8569
add More link to PRs #8342
Merge branch 'master' into oshmail-revamp
Merge branch 'master' into oshmail-revamp
Merge branch 'oshmail-revamp' of https://github.com/syslabcom/osha.theme into oshmail-revamp

Conflicts:
	src/osha/theme/cssselect.py