Updates to manifest storage

[davidfischer]

Edit 2018-08-01: I removed the actual storage backends. They will be used in a separate repository with deployment customizations. This PR is now relatively minor.

This builds on #4427. This can be merged into that branch or into master and that one can be closed.

• Because the manifest storage parses files looking for referenced files (eg. theme.css references fonts) all the referenced files must be there. This fixes a few issues found there.
• I do question whether readthedocs/core/storage.py should be in this repository and not in our extensions. I don't think it is particularly useful to anyone and is particular to our deploy process.
• This PR removes the references to this storage in readthedocs/settings/base.py. I think that in dev the default Django storage is fine. This should only be used in prod.
• There will still be a linting error with this PR as the linter will try to import from Azure and django-storages and fail. I can fix this up although as I mentioned above I think this code might be better elsewhere.

Testing

• Install dependencies

  $ pip install azure django-storages "azure-storage==0.20.3"
  

• Set settings (readthedocs/settings/base.py or other appropriate settings)

  STATICFILES_STORAGE = 'readthedocs.core.storage.MediaAzureStorage'
  AZURE_ACCOUNT_NAME = 'YOUR ACCOUNT'
  AZURE_ACCOUNT_KEY = 'YOUR KEY'
  AZURE_CONTAINER = 'media'
  AZURE_MEDIA_STORAGE_CONTAINER = AZURE_CONTAINER
  

• Collect static files

  $ npm run build      # takes about 10 minutes the first time and 2.5 mins thereafter
  

[ericholscher]

Yea, the settings change was just for testing. This can probably all just go in ext since it isn't useful for anyone else running RTD.

[davidfischer]

Ok. I'm going to remove the changes to readthedocs/core/storage.py from this PR. The other changes are still required if we want to use manifest storage.

[ericholscher]

👍 merging these since they are small simple fixes.