Allow news items to be linked to all versions of a project

[tompollard]

Currently, news items can only be linked to a single version of a published project, but in many or most cases we would actually like news items to link to all versions of a published project.

This pull request adds a flag to the news model that allows a news item to be linked to all versions of a project. Specifically, the changes are:

• Add a link_all_versions flag to the News model
• Allow the flag to be set in the form that is used to create news items
• Adds a get_all_news() method to the PublishedProject model that makes it straightforward to retrieve all news items for the project (both news items that are directly linked, and news items that are linked through the link_all_versions flag).
• Adds a migration that updates the News model.

There may be better ways to implement this functionality, e.g. by:

• linking the News model to a CoreProject instead of PublishedProject
• setting a validator that requires linked CoreProjects to be associated with a PublishedProject (so that news items can only be added for published projects)
• adding project_version to the News item
• updating the form used to add news items so that the CoreProject and version (or "all versions" can be set).

I would prefer to stick with the implementation in this pull request for timeliness (so that @briangow can respond to a user request). We can review and improve the approach in future.

[tompollard]

Thanks Brian

[tompollard]

@briangow This is now live. I've set link_all_versions to True for the 2021 announcements.

p = PublishedProject.objects.get(...)
for n in p.news.all():
    n.link_all_versions = True
    n.save()