Expose vocab information via REST API

[osma]

I think the Annif REST API could expose some more information about the vocabularies. Here is a list, ranging from easy to more difficult:

1. /projects/{projectId} could expose the vocabulary ID of a single project
2. /projects/ could expose the vocabulary IDs of all returned projects
3. there could be a /vocabs/ method that lists all vocabs along with basic information (similar to annif list-vocabs)
4. there could be a /vocabs/{vocabId} method that shows basic information about a single vocabulary
5. there could be additional methods that expose vocabulary concepts (but this is potentially getting into Skosmos territory...)

The reasons for implementing this:

• parity between the CLI and the REST API
• Finto AI, with the recent functionality for guessing the language, would benefit from knowing the vocabulary ID of a project - now it has to guess it from the project ID