Do not fetch the whole text file for the sidebar preview

[rullzer]

Just fetch the first 10kb. This should be more than enough in 99% of the
cases. And avoid downloading a 10mb text file just to display a tiny
portion.

Signed-off-by: Roeland Jago Douma roeland@famdouma.nl

[mention-bot]

@rullzer, thanks for your PR! By analyzing the history of the files in this pull request, we identified @icewind1991 and @MorrisJobke to be potential reviewers.

[MorrisJobke]

Tested and works 👍

[codecov]

Codecov Report

Merging #5954 into master will decrease coverage by 0.06%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##             master    #5954      +/-   ##
============================================
- Coverage     53.11%   53.05%   -0.07%     
  Complexity    22764    22764              
============================================
  Files          1404     1404              
  Lines         88145    88109      -36     
  Branches       1327     1327              
============================================
- Hits          46819    46747      -72     
- Misses        41326    41362      +36

Impacted Files	Coverage Δ	Complexity Δ
apps/federation/lib/AppInfo/Application.php	38.59% <0%> (-28.08%)	10% <0%> (ø)
apps/federation/lib/SyncJob.php	22.22% <0%> (-27.78%)	5% <0%> (+2%)
...deration/lib/BackgroundJob/RequestSharedSecret.php	50.74% <0%> (-15.49%)	19% <0%> (+2%)
...s/federation/lib/BackgroundJob/GetSharedSecret.php	49.38% <0%> (-13.96%)	22% <0%> (+3%)
apps/comments/lib/EventHandler.php	79.16% <0%> (-8.34%)	7% <0%> (ø)
lib/private/DB/QueryBuilder/QueryBuilder.php	84.65% <0%> (-3.89%)	62% <0%> (-3%)
...atenotification/lib/Controller/AdminController.php	63.79% <0%> (-2.88%)	11% <0%> (ø)
lib/private/Security/CertificateManager.php	91.08% <0%> (-1%)	39% <0%> (ø)
apps/federation/lib/DbHandler.php	95.86% <0%> (-0.43%)	22% <0%> (ø)
lib/private/legacy/defaults.php	42.99% <0%> (-0.28%)	45% <0%> (+1%)
... and 27 more