Support frameworks and manifests in ContentLoader

[robyoung]

As we move to handling multiple frameworks the ContentLoader needs to be able to support this. This commit changes the ContentLoader's role so that it is responsible for managing multiple manifests across multiple frameworks. Rather than creating multiple ContentLoader instances we create one, pointing it at the root content directory (digitalmarketplace-frameworks). On application startup we load the manifests we need and then get builders by framework slug and manifest name. If a manifest is not found either when loading or when getting a buidler we error with a ContentNotFound error and should fail fast.

Note that this changes read_yaml so that it raises an error if the file is not found. read_yaml only seems to be used by the content loader so this should not cause errors.

Example app change

At application startup

Old

existing_service_content = ContentLoader(
    'app/content/frameworks/g-cloud-6/manifests/edit_service.yml',
    'app/content/frameworks/g-cloud-6/questions/services/'
)
new_service_content = ContentLoader(
    'app/content/frameworks/g-cloud-7/manifests/edit_submission.yml',
    'app/content/frameworks/g-cloud-7/questions/services/'
)
declaration_content = ContentLoader(
    'app/content/frameworks/g-cloud-7/manifests/declaration.yml',
    'app/content/frameworks/g-cloud-7/questions/declaration/'
)

New

content_loader = ContentLoader('app/content')
content_loader.load_manifest('g-cloud-6', 'edit_service', 'services')
content_loader.load_manifest('g-cloud-7', 'edit_submission', 'services')
content_loader.load_manifest('g-cloud-7', 'declaration', 'declaration')

In the views

Old

content = declaration_content.get_builder()

New

content = content_loader.get_builder(framework_slug, 'declaration')

[allait]

I usually try to avoid defaultdicts since e.g. this will add a framework_slug key to _content whenever it's called with a missing slug. This might be not an issue for our use case, but I'd consider making load_manifest create framework keys explicitly

[quis]

Got this working locally in the supplier app. Didn't encounter any problems. I think the level of sophistication is about right when we're still dealing with a low number of frameworks.

[allait]

I'd go with framework_slug, question_set, manifest argument order, since manifests can only cover questions from one question set, and this would highlight the hierarchical order

Thinking about this some more, manifests are tied to the framework/question sets anyway, maybe the manifest file should declare this dependency instead of it being externally added by ContentLoader

[quis]

@allait I like your second point, eg changing the manifest from this:

-
  name: Page 1
  editable: True
  questions:
    - example

to something like this:

questions_path: declaration
sections:
  -
    name: Page 1
    editable: True
    questions:
      - example

Not sure if it's something for this PR or not.

[robyoung]

First point yes, I'll do that.

Second point yes, but I didn't want to do that as part of this PR.

[allait]

👍

✨ Changelog looks great! ✨