Avoid importing models in models

[funkybob]

Use lazy relation resolution for ForeignKey fields
Use django.db.models.get_model or django.apps.apps.get_model to get ContentType model in model methods.

[zhang-z]

@funkybob , thank you for the contribution.

I know lazy relation resolution is needed to avoid circular model imports in some cases. But importing models also has a benefit that if you use IDE's (e.g. Pycharm) refactoring feature to rename a model, the direct importing works better. In our case, circular model imports doesn't exist because notification app imports Django's builtin app while the reversed import won't happen. So is there any general discussion on why lazy relation resolution is preferred over importing models?

[funkybob]

Personally, I always use lazy references, but I don't use PyCharm, so don't know its limitations. However, I do agree a good discussion on the benefits is warranted, so have opened one on the django-developers mailing list.

I'm quite sure I was driven to that change to prevent app state not ready errors when migrating a project from 1.4 to 1.9... I will verify tomorrow.

[zhang-z]

@funkybob , thanks for opening the discussion. Let me paste the link here.

[zhang-z]

@funkybob , from the mailing list discussion, seems no more benefits of string FK references have been revealed, other than solving circular relations when needed. On the other side, regular FK references may help IDE to detect typos and dependencies. Do you think we can close this pull request until find more findings to support string FK references?