Rework templates into more simple/idiomatic jinja

While working on #34 (closed), I realized that a lot of work would be needed to move from HTML to markdown, due mostly to how templates are hacked together: most models have multiple template fields (_template, html, and body). Moving to markdown would be an immense amount of work, to the point where reworking the template system and then moving to markdown would probably be less work.

I'd like to make a series of small feature branches to migrate each model to a new layout.html template without breaking any un-migrated models, so that anyone working on these files concurrently isn't hit with a huge amount of conflicts all at once.

Hopefully the rework won't cause too much trouble, and the end result should have a lot less technical debt.

These files reference layout.html. If this all sounds good, I'll move the file to layout-legacy.html and point the references to that instead:

• ./templates/page.html:1:{% extends "layout.html" %}
• ./content/champions-of-privacy/contents.lr:5:_template: layout.html
• ./content/donate-thank-you/contents.lr:3:_template: layout.html
• ./content/contents.lr:13:_template: layout.html
• ./content/monthly-giving/contents.lr:13:_template: layout.html
• ./content/donor-faq/contents.lr:3:_template: layout.html
• ./content/privacy-policy/contents.lr:3:_template: layout.html
• ./content/state-disclosures/contents.lr:3:_template: layout.html
• ./content/cryptocurrency/contents.lr:3:_template: layout.html