Escape user-edited fields in OpenGraph meta tags

[HebaruSan]

Problems

• https://spacedock.info/pack/397/Cuinq
  
• https://spacedock.info/mod/1967 still crashes

Causes

• We need to handle HTML attribute values specially:
  • https://bleach.readthedocs.io/en/latest/clean.html

    If you need to use the output of bleach.clean() in an HTML attribute, you need to pass it through your template library’s escape function. For example, Jinja2’s escape or django.utils.html.escape or something like that.

  • https://jinja.palletsprojects.com/en/3.0.x/templates/#jinja-filters.escape
• Apparently ChainedUndefined (see Handle missing default mod version, don't close db before rendering templates #378) makes properties safe but not methods, and Add OpenGraph meta tags #385 added latest.created.strftime()

Changes

• Now the Jinja2 escape filter is applied to each instance of user-edited text in the og: meta tags, which will prevent them from terminating the attributes. bleach was mangling the text in a way that prevented escape from working, so I replaced bleach instead of combining them.
• Now if latest is None, we skip the whole first line of the mod description and just return Mod.short_description by itself
• Now if the user has a background set on his or her profile, it will be returned as the og:image rather than /static/og.png

Fixes #407.