Allow a submission to have multiple projects associated

[frjo]

Fixes #4869

Test Steps

• Tests that PROJECTS_ALLOW_MULTIPLE, default false, makes everything work as before, single project per submission.
• Test that setting PROJECTS_ALLOW_MULTIPLE to true allow multiple projects to be created.
• Test that multiple projects works with reports, invoices etc. Functionality within each project should be unchanged.
• Test that all project url:s work as expected. They are now in the form "apply/submissions/1234/project/4/". The last part, project id, was added to handle multiple projects per submission.

[wes-otf]

This is such an elegant solution! Did a pretty intensive test and it seems like everything is working as expected. The fact that I even changed MULTIPLE_PROJECTS_ENABLED=1 → MULTIPLE_PROJECTS_ENABLED=0 and nothing broke but preserved the multiple projects while not allowing multiples to be created moving forward was also really impressive. All the x.submission.id → x.pk changes look like they must've been painful hahaha.

Overall great work!

[wes-otf]

I like this format of kinda docstringing a template! super useful when trying to track down what the variables used are & where they come from