Add support for the new mixed method for power

[mrchrisadams]

This change adds support for a 'mixed' option when updating information about how datacenters and hosting providers are powered.

This is intended to allow for cases where a hosting company relies on multiple provider what might use different methods for accounting for their emissions.

It also accounts for datacentres who might use green energy when it's available (for example when they generate it themselves, but do not purchase things like RECs to account for energy used from the grid, where there is no scheme like GVOs or RECs, or similar).

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (master@ab7a36d). Click here to learn what that means.
The diff coverage is 80.00%.

@@            Coverage Diff            @@
##             master      #75   +/-   ##
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  Coverage          ?   57.13%           
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  Files             ?       53           
  Lines             ?     1605           
  Branches          ?      104           
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  Hits              ?      917           
  Misses            ?      682           
  Partials          ?        6           

Impacted Files	Coverage Δ
...pps/accounts/migrations/0009_auto_20200603_1109.py	0.00% <0.00%> (ø)
apps/accounts/models/choices.py	100.00% <100.00%> (ø)
apps/accounts/models/test_models.py	100.00% <100.00%> (ø)
apps/accounts/utils.py	27.77% <0.00%> (ø)
apps/greencheck/apps.py	0.00% <0.00%> (ø)
...pps/accounts/migrations/0007_auto_20200206_1105.py	0.00% <0.00%> (ø)
...ncheck/management/commands/dump_greenpresenting.py	0.00% <0.00%> (ø)
apps/accounts/migrations/0001_initial.py	0.00% <0.00%> (ø)
...anagement/commands/amend_hostingproviders_stats.py	0.00% <0.00%> (ø)
... and 46 more

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