Fix outdated Dataporten setup

[ddabble]

Proposed changes

Fixes

• Fixed logging out not working on subdomains other than the "main" subdomain (https://makentnu.no/) (cd5f89c91c9a272d3225b62855543411130324f2)
• Fixed logging out not automatically redirecting to the front page (b447c75890583f67cf3ec7bc16fb8346a7449993)

Other changes

• Updated the "social" config for authenticating with Dataporten (7830c20550af0bc30f25c873e0198c62152977a0)
  • See the commit message for more details
• Added some settings for the domain and security to use for session, CSRF and language cookies (357b6b4f4d0cb8679c4c936c511915f84d076ec5)

Areas to review closely

That logging in and out through Dataporten works as normal.

Deployment notes

Set the following settings in production:

• COOKIE_DOMAIN = ".makentnu.no" (note the prefixed .)
• COOKIE_SECURE = True

Also, the custom LOGOUT_URL setting can be removed, as well as the SESSION_COOKIE_DOMAIN setting (since it's set to the same value as COOKIE_DOMAIN).

Checklist

(If any of the points are not relevant, mark them as checked)

• [x] Created tests that fail without the changes, if relevant/possible
• [x] Made sure that your code conforms to the code style guides and that any common code smells have been addressed
  • (It's not intended that you read through this whole document, but that you get yourself an overview over its contents, and that you keep it in mind while taking a second look at your code before opening a pull request)
• [x] Added sufficient documentation - e.g. as docstrings or in the README, if suitable
• [x] Added your changes to the "Unreleased" section of the changelog - mainly the changes that are of particular interest to users and/or developers, if any
• [x] Added a "Deployment notes" section above, if anything out of the ordinary should be done when deploying these changes to the server

[ddabble]

Merging without explicit approval from another member, as the Dev committee agreed to merge these changes and assume they're relatively bug-free, simply to get things done quicker.