Feat/explicitly define help url for each rule

[Lakitna]

Closes #1123

This is a proof of concept for explicitly defining the help URL for each rule. This approach will require all default and community rules to be extended with a URL.

Fixes rule help URLs for community rules and custom rules. URLs are now defined per rule instead of globally.

This is a backward compatible change.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.93%. Comparing base (f62508c) to head (5fb7f77).
Report is 43 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1138      +/-   ##
==========================================
- Coverage   97.02%   96.93%   -0.09%     
==========================================
  Files          37       39       +2     
  Lines        4670     5030     +360     
==========================================
+ Hits         4531     4876     +345     
- Misses        139      154      +15     

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[Lakitna]

Still needs a bit of testing, but otherwise done :)

[Lakitna]

This PR is ready for review :)

[bhirsz]

I apologise for delay. I was conflicted with the topics because of the new ideas:

bhirsz/robotframework-cop-academy#33

I'm also refactoring defining the rules in new robocop heavily: https://github.com/bhirsz/robotframework-cop-academy/tree/feature/refactor_rules

That's why I was conflicted with introducing such breaking change, and then another. How do you feel about possibly postponing it, is it something you want sooner than later? Alternatively we can ignroe community rules for now (since they will be removed), and rename "DefaultRule" to "Rule" like in the beginning (sorry for the extra work). All the result may stay - in that case we will still have option to overwrite help url by creating own class but without any breaking change.