Don't warn users about *every* dirty form

[Treora]

The choice regarding which forms should or should not trigger a warning
is subjective. I tried to be consistent and not warn about forms that:

• run an action, rather than edit data: search, send an email.
• delete data: a warning about losing data would be confusing

Note that forms on sign-in pages were already ignored (using a selector,
rather than an explicit class on the form element).

Fixes #3698.

Note: I did not yet manage to build gitea (due to some problem with golang), so I have not been able to test this change properly

[codecov-io]

Codecov Report

Merging #3707 into master will increase coverage by <.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #3707      +/-   ##
==========================================
+ Coverage   35.79%   35.79%   +<.01%     
==========================================
  Files         287      287              
  Lines       41496    41496              
==========================================
+ Hits        14854    14855       +1     
  Misses      24454    24454              
+ Partials     2188     2187       -1

Impacted Files	Coverage Δ
models/repo_list.go	56.37% <0%> (-1.35%)	⬇️
modules/process/manager.go	81.15% <0%> (+4.34%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update e64aa18...1c3914f. Read the comment docs.

[lafriks]

@thehowl ~~~your suggestion does not help much ;)~~~

[thehowl]

@lafriks someone's pulling a prank on me, it seems (that's thehowi, not thehowl). Whatever twisted mind thought of that...