Make dashboard newsfeed list length a configurable item

[techknowlogick]

Fix #12466

[codecov-commenter]

Codecov Report

Merging #12469 into master will increase coverage by 0.02%.
The diff coverage is 50.00%.

@@            Coverage Diff             @@
##           master   #12469      +/-   ##
==========================================
+ Coverage   43.80%   43.82%   +0.02%     
==========================================
  Files         630      630              
  Lines       69821    69821              
==========================================
+ Hits        30583    30599      +16     
+ Misses      34301    34290      -11     
+ Partials     4937     4932       -5     

Impacted Files	Coverage Δ
modules/setting/setting.go	46.44% <ø> (ø)
models/action.go	51.74% <50.00%> (ø)
modules/charset/charset.go	68.53% <0.00%> (-2.25%)	⬇️
services/gitdiff/gitdiff.go	73.88% <0.00%> (-1.79%)	⬇️
models/issue.go	57.14% <0.00%> (+0.35%)	⬆️
modules/notification/webhook/webhook.go	48.95% <0.00%> (+0.52%)	⬆️
services/pull/pull.go	43.41% <0.00%> (+0.69%)	⬆️
modules/queue/workerpool.go	60.00% <0.00%> (+1.22%)	⬆️
services/pull/patch.go	68.96% <0.00%> (+2.58%)	⬆️
services/pull/check.go	57.03% <0.00%> (+3.12%)	⬆️
... and 2 more

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