Refactor image paste code

[silverwind]

Some minor refactors I did while investigating another issue. Functionality should be pretty much the same as before. New code should also handle pasting multiple images.

@a1012112796 I was unable to fix your paste bug. I think the bug is in CodeMirror itself and I was unable to prevent it from pasting the text alongside the image. It does work when you switch to <textarea/>, so this is another reason for #10729 and just go for <textarea/>.

[codecov-io]

Codecov Report

Merging #13354 into master will increase coverage by 0.00%.
The diff coverage is 0.00%.

@@           Coverage Diff           @@
##           master   #13354   +/-   ##
=======================================
  Coverage   42.12%   42.12%           
=======================================
  Files         689      689           
  Lines       75851    75853    +2     
=======================================
+ Hits        31949    31952    +3     
+ Misses      38670    38667    -3     
- Partials     5232     5234    +2     

Impacted Files	Coverage Δ
routers/api/v1/repo/issue_reaction.go	46.76% <0.00%> (-0.69%)	⬇️
modules/charset/charset.go	68.53% <0.00%> (-4.50%)	⬇️
modules/queue/workerpool.go	60.00% <0.00%> (-2.05%)	⬇️
modules/queue/unique_queue_disk_channel.go	53.84% <0.00%> (-1.54%)	⬇️
services/pull/patch.go	55.68% <0.00%> (+1.70%)	⬆️
modules/log/event.go	61.79% <0.00%> (+1.88%)	⬆️
services/pull/check.go	51.82% <0.00%> (+2.91%)	⬆️
services/pull/temp_repo.go	29.78% <0.00%> (+3.19%)	⬆️

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[techknowlogick]

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