[stable23] Make user agent configurable by admin

[backportbot-nextcloud[bot]]

backport of #300

[codecov[bot]]

Codecov Report

Merging #302 (1d86c0d) into stable23 (d36a1dd) will not change coverage. The diff coverage is n/a.

@@             Coverage Diff             @@
##             stable23     #302   +/-   ##
===========================================
  Coverage       94.54%   94.54%           
  Complexity         40       40           
===========================================
  Files               3        3           
  Lines             110      110           
===========================================
  Hits              104      104           
  Misses              6        6           

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