Make URLs end with a trailing slash + replace some hardcoded URLs with lookups

[ddabble]

Made most of the project's URLs end with a trailing slash (0ba27aa8155d1029edcee6e0895ffef0109bf8c3).

Reasons:

• Many/most of the website's views use a trailing slash in their URLs, so this will make it consistent and (hopefully) establish an internal convention
• To my knowledge, it's conventional for (Django) websites to use a trailing slash in URLs (except for URLs to files, of course)
• Django redirects URLs missing a trailing slash to the same URL with a trailing slash by default, but not vice versa (see the docs on the APPEND_SLASH setting)
• This fixes #806

Also:

• Replaced hardcoded success_urls with lookups (8b20b52ef5328dc0efdb6fd4d8659d7dd3bc6cf9)
• Replaced hardcoded hrefs with lookups (aa22ea01d4f5e78fbcf3eb1a4d0fbde102e3fcf6)

Note that I've not taken the time to test the code after making these changes (other than running the tests, which don't seem to cover most of the URLs), but I have been thorough in checking the whole codebase for references to the old URLs. Nonetheless, I'd appreciate if one of the maintainers could help me catch any URLs I might have missed :)

[michaelbrusegard]

Fixed the merge conflict in a new branch, because I can not modify a branch outside of this repo, see #810. Good work!

[ddabble]

Fixed the merge conflict in a new branch

Nice, thanks 😊

because I can not modify a branch outside of this repo

Thought I'd let you know that, actually, you can, since I (and most people when creating a PR from a fork) left the "Allow edits and access to secrets by maintainers" checkbox checked when creating the PR, which allows maintainers to e.g. rebase and push to that specific branch 🙂