[master] Configurable thumbnail path

[PVince81]

Forward port of #35131 to master

[mmattel]

Many thanks for adding that 👍

[phil-davis]

https://drone.owncloud.com/owncloud/core/17263/869

git fetch --no-tags --depth=50 origin +refs/pull/35196/merge:
fatal: expected shallow/unshallow, got ERR upload-pack: not our ref e76734932290d039a1f4f473fef1874d26c87e8a
fatal: the remote end hung up unexpectedly
time="2019-05-10T07:07:06Z" level=fatal msg="exit status 128"

some annoying github timing thing
I will restart drone

[codecov]

Codecov Report

Merging #35196 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@             Coverage Diff              @@
##             master   #35196      +/-   ##
============================================
+ Coverage      65.5%    65.5%   +<.01%     
- Complexity    18640    18644       +4     
============================================
  Files          1217     1218       +1     
  Lines         70554    70565      +11     
  Branches       1296     1296              
============================================
+ Hits          46213    46224      +11     
  Misses        23964    23964              
  Partials        377      377

Flag	Coverage Δ	Complexity Δ
#javascript	53.5% <ø> (ø)	0 <ø> (ø)	⬇️
#phpunit	66.89% <100%> (ø)	18644 <4> (+4)	⬆️

Impacted Files	Coverage Δ	Complexity Δ
lib/private/Files/Mount/PreviewsMountProvider.php	100% <100%> (ø)	4 <4> (?)
lib/private/Server.php	86.57% <100%> (+0.01%)	253 <0> (ø)	⬇️
lib/private/Files/Mount/CacheMountProvider.php	100% <100%> (ø)	4 <0> (ø)	⬇️

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