Add support for external translation memories

**Is your feature request related to a problem? Please describe.**
Current built in translation memory works badly in many setups. It might improve once we migrate it to the database, but proper translation memory service might still provide better results.

**Describe the solution you'd like**
It might be better to interface with third-party translation memories to provide better results.

* https://github.com/translate/amagama/
* http://www.opentm2.org/ / https://github.com/OpenTM2/opentm2-source

**Describe alternatives you've considered**
Developing full feature translation memory in Weblate is also an option.