Secrets only use one language

[ddabble]

Proposed changes

Secrets now only use one language, for the same reason that the internal home content box only uses one language - namely that it's less of a hassle to fill in one instead of two languages, and that all MAKE members are required to be able to understand written Norwegian. Also, several secrets on the live site either had their Norwegian contents copied to the English contents, or they only had Norwegian content with the English content simply saying to see the Norwegian page.

Some additional improvements:

• Content boxes cannot be changed on other subdomains (33ee899b693ef837e4a91c3103a287deab19a4c3)

Areas to review closely

None in particular.

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 guide 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

[codecov[bot]]

Codecov Report

Merging #457 (4c52315) into dev (17766a9) will increase coverage by 0.00%. The diff coverage is 100.00%.

@@           Coverage Diff           @@
##              dev     #457   +/-   ##
=======================================
  Coverage   87.95%   87.95%           
=======================================
  Files         134      134           
  Lines        5388     5390    +2     
=======================================
+ Hits         4739     4741    +2     
  Misses        649      649           

Impacted Files	Coverage Δ
contentbox/forms.py	100.00% <100.00%> (ø)
contentbox/views.py	100.00% <100.00%> (ø)
internal/models.py	93.19% <100.00%> (ø)
web/multilingual/admin.py	91.66% <100.00%> (+0.36%)	:arrow_up:
web/multilingual/data_structures.py	95.45% <100.00%> (ø)
web/multilingual/formfields.py	94.33% <100.00%> (ø)
web/multilingual/modelfields.py	100.00% <100.00%> (ø)
web/multilingual/widgets.py	100.00% <100.00%> (ø)

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