(Re)Load issue labels when changing them

[zeripath]

(Re)Load issue labels when labels are added or removed

This means that the label state that the webhooks produce accurately matches
the changes that they are reporting.

Fix #10660

Signed-off-by: Andrew Thornton art27@cantab.net

[zeripath]

(I have confirmed that this works for the testcase on the associated PR)

[codecov-commenter]

Codecov Report

Merging #13007 into master will increase coverage by 0.00%.
The diff coverage is 33.33%.

@@           Coverage Diff           @@
##           master   #13007   +/-   ##
=======================================
  Coverage   42.58%   42.58%           
=======================================
  Files         671      671           
  Lines       73632    73644   +12     
=======================================
+ Hits        31358    31364    +6     
- Misses      37188    37191    +3     
- Partials     5086     5089    +3     

Impacted Files	Coverage Δ
models/issue.go	56.58% <33.33%> (-0.07%)	⬇️
models/issue_label.go	67.15% <33.33%> (-0.76%)	⬇️
modules/log/event.go	57.54% <0.00%> (-1.89%)	⬇️
modules/queue/unique_queue_disk_channel.go	53.84% <0.00%> (-1.54%)	⬇️
modules/queue/workerpool.go	58.77% <0.00%> (-1.23%)	⬇️
models/gpg_key.go	53.33% <0.00%> (-0.58%)	⬇️
models/error.go	34.85% <0.00%> (+0.50%)	⬆️
modules/git/repo.go	46.70% <0.00%> (+0.50%)	⬆️
models/unit.go	49.31% <0.00%> (+2.73%)	⬆️
modules/git/utils.go	77.04% <0.00%> (+3.27%)	⬆️
... and 2 more

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