Delete and unload model from server through API

[bigman73]

Feature request

rasa NLU version (e.g. 0.7.3):
0.8.10

Used backend / pipeline (mitie, spacy_sklearn, ...):
spacy_sklearn

Operating system (windows, osx, ...):
Docker/Linux

Issue:
Add an REST DELETE API for deleting an available model. As of 0.8.10 the workaround is to delete the trained model folder from the parent /models folder.
The Delete API should not just delete the folder but clean up any internal data structures holding the model trained data - basically untrain the model.

Content of configuration file (if used & relevant):

[tmbo]

That sounds like a good addition 👍