WIP: Add configuration file

[wojas]

What is the purpose of this change? What does it change?

This implements a YAML config file mechanism for rest-server. See #139 for details.

Was the change discussed in an issue or in the forum before?

Closes #139 (config file)
Closes #94 (TLS hot reload)
Closes #74 (per-user config overrides)
TODO: Closes #73 (TLS client auth): still need to actually use the client TLS info

Try it out

To build this branch, you need git and a recent Go compiler installed and run the following:

git clone -b config-file https://github.com/wojas/rest-server.git
cd rest-server
go build -o rest-server ./cmd/rest-server

You will then have a rest-server binary in this directory.

Checklist

• I have enabled maintainer edits for this PR
• I have added tests for all changes in this PR
• I have added documentation for the changes (in the manual)
• There's a new file in changelog/unreleased/ that describes the changes for our users (template here)
• I have run gofmt on the code in all commits
• All commit messages are formatted in the same style as the other commits in the repo
• I'm done, this Pull Request is ready for review