Flagging improvements

[Oaphi]

closes #1289

This PR includes the following improvements to flagging:

• flags are now validated for max length (see linked issue) - configurable via a new site setting MaxFlagReasonLength;
• flag creation messages have been moved to be provided by the server;
• we no longer rely on jQuery for raising flags;
• we now have locale strings for flag actions (Rails must be restarted for it to be picked up) - only flags.errors.rate_limited is added for now, though, to keep the PR simple;

[codecov]

Codecov Report

❌ Patch coverage is 88.88889% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 74.89%. Comparing base (2ae9944) to head (4965b39).
⚠️ Report is 18 commits behind head on develop.

Files with missing lines	Patch %	Lines
app/controllers/flags_controller.rb	75.00%	1 Missing ⚠️

Additional details and impacted files

Components	Coverage Δ
controllers	70.03% <75.00%> (+0.17%)	⬆️
helpers	81.82% <ø> (ø)
jobs	60.00% <ø> (ø)
models	86.17% <100.00%> (+0.04%)	⬆️

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[cellio]

Tested with <, =, and > limit and works as expected. Most of the code makes sense to me (do get a more qualified reviewer), though the ...flag part on line 458 in the JS file I don't understand at all (my failing not yours). I left a couple comments asking about ways to improve the user feedback; if the answer is "not now" and that would be a separate issue later, that's fine. But if it's easy, can we try to do something here?

[cellio]

Approved behavior/tests; someone else should also review.