[10.4] Read only mount

[phil-davis]

This is a "backport" of #36397 to the release-10.4 branch.

Hopefully this can get into 10.4

See the original master PR for details

[codecov]

Codecov Report

Merging #36680 into release-10.4.0 will decrease coverage by <.01%.
The diff coverage is 57.14%.

@@                 Coverage Diff                  @@
##             release-10.4.0   #36680      +/-   ##
====================================================
- Coverage             64.68%   64.68%   -0.01%     
- Complexity            19121    19122       +1     
====================================================
  Files                  1269     1269              
  Lines                 74771    74778       +7     
  Branches               1320     1320              
====================================================
+ Hits                  48368    48372       +4     
- Misses                26015    26018       +3     
  Partials                388      388

Flag	Coverage Δ	Complexity Δ
#javascript	54.12% <ø> (ø)	0 <ø> (ø)	⬇️
#phpunit	65.85% <57.14%> (-0.01%)	19122 <0> (+1)

Impacted Files	Coverage Δ	Complexity Δ
apps/files_external/js/settings.js	51.84% <ø> (ø)	0 <0> (ø)	⬇️
apps/files_external/lib/Command/ListCommand.php	68.04% <0%> (-0.41%)	0 <0> (ø)
apps/files_external/templates/settings.php	33.69% <100%> (+0.72%)	0 <0> (ø)	⬇️
lib/private/legacy/util.php	73.59% <60%> (-0.12%)	219 <0> (+1)

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