Rename investment phase to stage

[reupen]

For consistent use of terminology throughout the app.

The investment endpoints still accept and return phase for backwards compatibility.

[codecov-io]

Codecov Report

Merging #288 into develop will increase coverage by <.01%.
The diff coverage is 100%.

@@             Coverage Diff             @@
##           develop     #288      +/-   ##
===========================================
+ Coverage    92.92%   92.92%   +<.01%     
===========================================
  Files           63       63              
  Lines         2417     2418       +1     
  Branches       212      212              
===========================================
+ Hits          2246     2247       +1     
  Misses         145      145              
  Partials        26       26

Impacted Files	Coverage Δ
datahub/investment/views.py	93.42% <ø> (ø)	⬆️
datahub/metadata/views.py	100% <ø> (ø)	⬆️
datahub/search/views.py	98.57% <ø> (ø)	⬆️
datahub/metadata/admin.py	100% <ø> (ø)	⬆️
datahub/search/elasticsearch.py	98.3% <ø> (ø)	⬆️
datahub/investment/serializers.py	99.02% <100%> (ø)	⬆️
datahub/core/constants.py	100% <100%> (ø)	⬆️
datahub/investment/validate.py	100% <100%> (ø)	⬆️

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