Add basic read-only replica handling

[mumblepins]

Implements basic handling of a database (repositories and indices) that are read only, intended to work as a read only replica.

Example:

# primary runner, pulls updates and runs indexing
docker run -d --rm --name hound-primary \
  -v $PWD/config.json:/data/config.json:ro \
  -v $PWD/db:/data/db -p 6080:6080 hound

# read only replica, watches for changes to repositories and looks for updated index

docker run -d --rm --name hound-ro \
  -v $PWD/config.json:/data/config.json:ro \
  -v $PWD/db:/data/db:ro -p 6081:6080 hound

What kind of change does this PR introduce? (check at least one)

• Bugfix
• Feature
• Code style update
• Refactor
• Build-related changes
• Other, please describe:

The PR fulfills these requirements:

• All tests are passing?
• New/updated tests are included?
• If any static assets have been updated, has ui/bindata.go been regenerated?
• Are there doc blocks for functions that I updated/created?

If adding a new feature, the PR's description includes:

• A convincing reason for adding this feature (to avoid wasting your time, it's best to open a suggestion issue first and wait for approval before working on it)