CI: Update GitHub Actions config for uniformity

[orangejulius]

This updates CI configuration for this repo with the following changes:

• the core configuration for running unit tests and integration tests are moved to separate files for reuse
• a new pull request workflow is added that only runs on external forks, and runs unit/integration tests
• Ubuntu versions are now pulled from a GitHub Actions variable (see https://github.com/pelias/pelias/issues/951)