Add endpoints for source and translation string details.

This adds 3 endpoints: source strings, translation strings, and both.
The endpoints use query parameter 'ids' to request a specific set of
translation units using the textflow numeric id (comma-separated list).

This also replaces the simple list of path strings with a list of thunks,
allowing far more flexibility in specifying paths, particularly sets
of similar paths that would require a lot of repetition.