Introduction of runnable base class to assist with introduction of fitting specs

[sroderick-g5sro]

Part of #134

[sroderick-g5sro]

Hi @jonc125. I am submitting this pull request for your review but I am aware of a number of issues:

1. I am not sure that I have taken the best course through the migrations and there maybe a number of redundant steps.
2. In one step renaming ExperimentVersion to Runnable meant that I had to change all references in the code to generate the automatic migration. In particular I had to remove code in experiments/admin.py I have readded that and reverted the global reference changes. These steps didn't "feel" right and there may have been a simpler approach.
3. In migration 0028_auto_20200212_1701.py I have added the logic to migrate the experiment_version records. I haven't added a reverse step to remove these - which I am guessing is required? Also I probably need to rename the file to something other than auto.
4. I have fixed the resulting unit test failures for example when fetching related experiments I have changed the code from experiment_version__experiment to experiment_version__experimentversion__experiment. I am assuming that I can do this double hop as the test pass.
5. There is one test failure that I am still working on. Experiment>test_sends_mail_when_experiment_is_finished

In general it would be good to get your feedback on the approach I have taken in this change please

[jonc125]

This was a complicated one to get right! I've worked through the migrations, and think I can see a pathway to getting it working. See what you think of my suggestions.

[codecov-io]

Codecov Report

Merging #253 into master will decrease coverage by <.1%.
The diff coverage is 92.8%.

@@           Coverage Diff            @@
##           master    #253     +/-   ##
========================================
- Coverage    94.5%   94.5%   -0.1%     
========================================
  Files          65      65             
  Lines        2685    2689      +4     
  Branches      280     280             
========================================
+ Hits         2540    2543      +3     
- Misses        105     106      +1     
  Partials       40      40

Impacted Files	Coverage Δ
weblab/experiments/views.py	98.3% <ø> (ø)	⬆️
weblab/conftest.py	99.3% <100%> (ø)	⬆️
weblab/experiments/apps.py	100% <100%> (ø)	⬆️
weblab/experiments/signals.py	100% <100%> (ø)	⬆️
weblab/experiments/emails.py	100% <100%> (ø)	⬆️
weblab/core/recipes.py	100% <100%> (ø)	⬆️
weblab/entities/views.py	93% <100%> (ø)	⬆️
weblab/experiments/models.py	98.1% <87.5%> (-1%)	⬇️
weblab/experiments/processing.py	92.5% <90%> (ø)	⬆️

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