Fix: Restore media search behaviour

[fosterfarrell9]

As described in #496, PR #420 has introduced some unwanted behaviour (that also leads to exceptions) when the media search was made accesible to generic users via the start page. In this PR the following things have been changed

• In the 'Import Content' tab in the 'Create vertex' dialogue for quizzes, there is no more radio button that allows you to select all types of media. You can only choose between Questions and Remarks. (This restores previous behaviour)
• In the 'Media Search' Tab that is available to editors in the admin area, it is now also possible to select media of type Questions/Remark/Erdbeere. (This restores previous behaviour)
• In the media search form on the start page, checking the 'all types' radio button now means checking all types of media actually listed in this generic search form, which excludes e.g. Questions. For generic users these types have been filtered out anyway if you selected 'all', but for admins they were not, creating an inconsistency. I think that the exclusion of certain types makes sense in that form, because the results are rendered in a way that is not suitable for huge amounts of hits. If you are an editor/admin and want to search Questions, you can use the search form in the admin area. Maybe it would make sense to add a helpdesk in this accordion fold for editors that explains that this media search is kind of restricted and that it is recommended to use the media search in the admin area. (This removes an inconsistency)

[codecov]

Codecov Report

Merging #506 (cb40501) into mampf-next (6ecf8a4) will increase coverage by 0.01%.
The diff coverage is 72.72%.

@@              Coverage Diff               @@
##           mampf-next     #506      +/-   ##
==============================================
+ Coverage       66.48%   66.50%   +0.01%     
==============================================
  Files             311      311              
  Lines            9406     9412       +6     
==============================================
+ Hits             6254     6259       +5     
- Misses           3152     3153       +1     

Impacted Files	Coverage Δ
app/controllers/media_controller.rb	25.87% <ø> (ø)
app/helpers/media_helper.rb	32.46% <0.00%> (-0.43%)	⬇️
app/models/medium.rb	54.85% <88.88%> (+0.19%)	⬆️

... and 1 file with indirect coverage changes