Add light theme template and other minor tweaks

[azrafe7]

This PR:

• adds up/down arrows (base64 encoded) to the old.reddit template
• makes the old.reddit template self contained (only index.html, snipped.css inlined, so that now works properly with jinja)
• adds settings.template_abbreviated_style (more on that later)
• adds a light_reddit_mockup template, based on the dark one

settings.template_abbreviated_style changes how the computation of timedelta and vote count strings are displayed ('1.1k' vs '1.1K', '4 days ago' vs '4d ago', etc.)

Old templates screenshots:

New templates screenshots:

[alexlaverty]

I've fixed up the PR build check now as well, checks should build properly now