WIP: Add history

[perllaghu]

This is working code, but has no tests.

I've built the notebook extensions in the same way that the nbgrader extensions were build.

There's also a couple of features I need to finish off - but the code's good for review, and improvement.

Also need to consider a commandline "app" for this!

[codecov]

Codecov Report

Merging #54 (21d7fa7) into master (214e6de) will decrease coverage by 2.05%.
The diff coverage is 47.83%.

@@            Coverage Diff             @@
##           master      #54      +/-   ##
==========================================
- Coverage   88.46%   86.40%   -2.06%     
==========================================
  Files          73       76       +3     
  Lines        6278     6460     +182     
==========================================
+ Hits         5554     5582      +28     
- Misses        724      878     +154     

Impacted Files	Coverage Δ
nbexchange/alembic/env.py	75.00% <ø> (ø)
...versions/d500457efb3b_create_nb_exchange_tables.py	51.85% <ø> (ø)
...s/f3345539f08d_change_assignment_name_data_type.py	66.66% <ø> (ø)
nbexchange/plugin/fetch_assignment.py	82.27% <ø> (ø)
nbexchange/plugin/release_assignment.py	90.16% <ø> (ø)
...e/server_extensions/nbexchange_history/__init__.py	0.00% <0.00%> (ø)
...e/server_extensions/nbexchange_history/handlers.py	0.00% <0.00%> (ø)
nbexchange/tests/test_handlers_feedback.py	95.25% <0.00%> (ø)
nbexchange/tests/test_plugin_fetch_assignment.py	96.42% <ø> (ø)
nbexchange/handlers/history.py	23.21% <23.21%> (ø)
... and 32 more

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