Federated translation memory

**Is your feature request related to a problem? Please describe.**
It might be interesting to share translation memory between instances. For example Fedora and openSUSE might be interested in this, but it's more generic topic.

**Describe the solution you'd like**
The design is still unclear:

* Run centralized service which all projects would use, this might utilize amaGama and https://github.com/WeblateOrg/weblate/issues/3561
* Add federation support directly to Weblate translation memory, maybe using https://en.wikipedia.org/wiki/ActivityPub