Allow multiple phases per challenge

[jmsmkn]

This PR:

• Breaks the 1:1 relationship between evaluation configurations and challenges
• Renames Config to Phase
• Links Method and Submission to Phase rather than Challenge

This PR does not allow for multiple phases per challenge yet, but is the first of 3 steps in doing so.

#738

[codecov]

Codecov Report

Merging #1488 into master will increase coverage by 0.00%.
The diff coverage is 97.66%.

@@           Coverage Diff           @@
##           master    #1488   +/-   ##
=======================================
  Coverage   94.46%   94.47%           
=======================================
  Files         598      599    +1     
  Lines       22695    22715   +20     
=======================================
+ Hits        21439    21459   +20     
  Misses       1256     1256           

Impacted Files	Coverage Δ
app/config/settings.py	94.28% <ø> (ø)
app/grandchallenge/challenges/tasks.py	0.00% <ø> (ø)
app/grandchallenge/datasets/models.py	96.15% <ø> (ø)
app/grandchallenge/evaluation/urls.py	100.00% <ø> (ø)
app/grandchallenge/statistics/views.py	100.00% <ø> (ø)
app/tests/core_tests/test_fixtures.py	100.00% <ø> (ø)
app/tests/factories.py	100.00% <ø> (ø)
app/grandchallenge/evaluation/views.py	79.53% <75.00%> (ø)
app/grandchallenge/evaluation/models.py	90.70% <91.66%> (+0.08%)	⬆️
...e/challenges/management/commands/copy_challenge.py	100.00% <100.00%> (ø)
... and 21 more

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