[stable12] theme flow redirection page

[MorrisJobke]

Backport of #7110 because we use it more and more in the android app as well ;)

Before:

After:

[codecov]

Codecov Report

Merging #7114 into stable12 will increase coverage by <.01%.
The diff coverage is 100%.

@@              Coverage Diff               @@
##             stable12    #7114      +/-   ##
==============================================
+ Coverage       53.76%   53.76%   +<.01%     
  Complexity      22585    22585              
==============================================
  Files            1384     1384              
  Lines           86655    86655              
  Branches         1329     1329              
==============================================
+ Hits            46588    46589       +1     
+ Misses          40067    40066       -1

Impacted Files	Coverage Δ	Complexity Δ
core/templates/loginflow/redirect.php	0% <ø> (ø)	0 <0> (ø)	⬇️
core/Controller/ClientFlowLoginController.php	78.87% <100%> (ø)	20 <0> (ø)	⬇️
lib/private/Server.php	84.68% <0%> (-0.13%)	121% <0%> (ø)
apps/files_trashbin/lib/Trashbin.php	72.53% <0%> (+0.24%)	136% <0%> (ø)	⬇️
lib/private/Security/CertificateManager.php	92.07% <0%> (+0.99%)	39% <0%> (ø)	⬇️