[stable12] {J,CS}SResourceLocator: account for symlinks in app path

[kyrofa]

Currently, if the app path includes a symlink, the calculated webDir will be incorrect when generating CSS and URLs will be pointing to the wrong place, breaking CSS.

This PR is a backport of #7061, fixing #6028 for stable12 by using realpath when retrieving app path, which makes these issues go away.

[codecov]

Codecov Report

Merging #7170 into stable12 will increase coverage by 0.03%.
The diff coverage is 66.66%.

@@              Coverage Diff               @@
##             stable12    #7170      +/-   ##
==============================================
+ Coverage       53.76%   53.79%   +0.03%     
- Complexity      22587    22588       +1     
==============================================
  Files            1384     1384              
  Lines           86660    86663       +3     
  Branches         1329     1329              
==============================================
+ Hits            46593    46621      +28     
+ Misses          40067    40042      -25

Impacted Files	Coverage Δ	Complexity Δ
lib/private/Template/CSSResourceLocator.php	0% <0%> (ø)	27 <0> (ø)	⬇️
lib/private/Template/JSResourceLocator.php	50.9% <100%> (+50.9%)	23 <0> (+1)	⬆️
lib/private/Files/Cache/Propagator.php	94.93% <0%> (-1.27%)	16% <0%> (ø)
lib/private/Template/ResourceLocator.php	66.66% <0%> (+1.85%)	20% <0%> (ø)	⬇️

[kyrofa]

@juliushaertl this review is identical to the one you've already done. Would you mind taking a quick look when you have a moment?

[pipiche38]

This issue seems to still be in 12.0.4
Here is an extract of my config file and if the app is stored on the /var/lib/nextcloud/apps directory - and not on the /usr/share/nextcloud/apps - I got the issue
'apps_paths' =>
array (
0 =>
array (
'path' => '/usr/share/nextcloud/apps',
'url' => '/apps',
'writable' => false,
),
1 =>
array (
'path' => '/var/lib/nextcloud/apps',
'url' => '/apps-appstore',
'writable' => true,
),
),