feat(medcat-trainer): Speed up project upload

[mart-r]

This PR attempts to speed up project upload.

I was having trouble uploading the entity linking challenge project (i.e conversion from the raw linking challenge) to the app.cogstack.org trainer. It seemed to just crash it.
However, when I tried on the demo trainer (sites.er.kcl.ac.uk), it worked just fine.

So I thought perhaps the app.cogstack.org is timing out.

Looking at the timings, it looked like it was taking around 5m30s to upload the dataset to the the trainer instance I was running locally.
I got the time down to around half.
More specifically, it took 328.675s before, and with the changes it took 150.610s.

Here's the changes:

• Avoid resaving the ProjectAnnotateEntities twice
  • The signal caused every save to happen twice for the entire DB entry
• Avoid some file system IO by caching CUI filter file
  • This was also loaded off disk at every save time
• Use model update instead of save for modifcation date of project annotate entities
  • The idea being that instead of updating all fields, only the relevant field gets updated

EDIT:
The next step would be to run this as a background task. But then some of the UI would need to also reflect that. So didn't get a chance to do that yet.