Replace Handlebars with Tera

[senekor]

This is very similar to #688, with some exceptions:

• I used snapshot testing over the entire site/ directory to ensure the output is identical.
  The snapshot testing setup is on the branch snapshot_tests.
  If this PR is approved, I will rebase the commits on the master branch to drop the testing setup.
• I made a separate commit for renaming the template files.
  Viewing the diff of the individual commits should make it easier to review.
• Tera and Handlebars don't handle HTML escaping exactly the same way, so I added the custom filter escape_hbs to ensure identical output.
  This could probably be removed later without issues.
• Using macros for headers, nav and footer is not necessary, an include statement would've been more concise.
  But the indent filter only works with macros and it allows identical output and a good diff at the same time.

[Manishearth]

• I used snapshot testing over the entire site/ directory to ensure the output is identical.

thank you so much! this is super valuable.

It might be worth having a long term snapshot testing setup checked in to the tree. We won't check in snapshots, but it will allow people to perform these tests conveniently

[senekor]

It might be worth having a long term snapshot testing setup checked in to the tree. We won't check in snapshots, but it will allow people to perform these tests conveniently

Alright, I will try to make a PR with a tweaked setup so it can be merged for convenience without getting in the way.

[JoHaHu]

This seems to have broken the rss feed.

[senekor]

oof, thanks for the report. I had that excluded from the the snapshot tests because it was non-deterministic with its updated timestamp 🙁 gonna fix this right away

Edit: should be fixed by #1505