Download template from thunderbird-website for a more up-to-date copy

[MelissaAutumn]

(Fixes #24)

Changes

• Added -r/--refresh_template argument for preview.py
• Templates are downloaded when --refresh_template is called, or if the release-notes file doesn't exist
• Templates are downloaded into _template/
• Some template includes are stubbed out
• Added requests & deps in requirements.txt
• Updated pyyaml, watchdog, and markupsafe
• Removed old template files

Known issues

• Fonts are hit by CORS.
• The new site design uses inline svgs a lot, which doesn't work well with img linking them
• THUNDERBIRD_URL is currently pointed to new.thunderbird.net

We can pull in font files and download and inline svgs as well if you'd like. And THUNDERBIRD_URL will be swapped to stage once the design is finally approved for stage/prod.

Let me know if you have any issues with the design or would like to request adjustments. (Feel free to ping me for those as well.)

(Comparison at: https://new.thunderbird.net/en-US/thunderbird/125.0beta/releasenotes/)