RMST-364 - Implementing cache-control header for git-reader routes

[alexcottner]

RMST-364

Adding cache rules to git-reader. This works similarly to what we have in the v1 readers, but I tried to simplify it into the main cache rules that we have.

• Volatile content that frequently changes (broadcasts, preview data, frequently changed endpoints, etc). Default to 60 seconds ttl.
• Stable content that rarely changes (changesets, hello, etc). Default to 1 hour ttl.
• Static content that doesn't change (attachments, cert-chains). Default to 1 week ttl.

Uses our existing Settings pattern.
Updated documentation.

[leplatrem]

Looks good 👌

While reading the diff, I was thinking we could have a helper like add_cache_header(response, settings, "long") but it adds one indirection and I don't think it's justified

Regarding the middleware, I think it's the right pattern. Setting the env var with monkeypath is the only way if we want to set it to tempdir

[leplatrem]

We currently list settings in the README, maybe we could remove them and keep deployment.rst only

[alexcottner]

Good call. Also realizing that our formatting in deployment.rst was slightly off. Fixing formatting and adjusting read-me to link to the latest documentation.