Protect master branch and enforce PR review + CI

[hannahhoward]

The reality is we've had some terrible issues with updates breaking the app in production. While I don't want to add too much process, as autoretrieve's stability is increasingly critical to at least Bedrock's usage, we need to put some safegaurds in place.

I am proposing the following, once CI is setup:

• no pushes direct to master except by admins (and admins only do this in cases of special exception)
• always push branches and PR when you want to make changes
• PRs must pass CI and have 1 approval prior to merge (admins can override, but generally, don't do this)

[anjor]

@hannahhoward i have enabled requiring PRs.

I wasn't sure what status checks you wanted enabled, so haven't done that yet.