Persist JSON-LD Contexts

**Issue**
JSON-LD contexts are cached in memory but not persisted.

In case a JSON-LD context resource is not available (e.g. the host went down), the server cannot parse json-ld objects with the given context.

**Proposal**
We should persist JSON-LD contexts (e.g. in redis / moleculer cache) or perhaps hard-code certain ones, as done with the ActivityStreams conetxt.

No files, tests, or entry points are named. First locate the in-memory JSON-LD context cache and the server's JSON-LD parsing path, then compare Redis or Moleculer cache persistence with hard-coded contexts; done means parsing still works when the remote context resource is unavailable.

Written by the indexing model from the issue text.