feat(general): Reject non http/https in help_link

[mitsuhiko]

This rejects non http/https links in the system for help_link. Refs getsentry/sentry#12659

[codecov-io]

Codecov Report

Merging #192 into master will increase coverage by 0.06%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #192      +/-   ##
==========================================
+ Coverage   65.26%   65.33%   +0.06%     
==========================================
  Files          84       84              
  Lines        9130     9148      +18     
==========================================
+ Hits         5959     5977      +18     
  Misses       3171     3171

Impacted Files	Coverage Δ
general/src/store/normalize/mechanism.rs	36.17% <100%> (+1.67%)	⬆️
general/src/store/normalize.rs	93.13% <100%> (ø)	⬆️

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