feat(settings): custom shortcuts

[noirbizarre]

This PR is based on and includes #1940

It makes the prefixes (currently gh: and gl:) customizable, allowing to:

• add your own (remote and local)
• override the existing ones (especially to force ssh access on private repositories)
• benefit from specific github and gitlab behavior on alternative, private or corporate instances

It moves URL normalization into the settings to benefit from custom shortcuts.

It is a draft has some info and decisions are required on the vcs.get_repo() behavior:
it currently only assumes https://gitlab and https://github are http git repositories, unless prefixed with git+.
However, non git remotes are not supported, so I wonder if it would be simpler to consider all https:// as git repositories (it would fail if this is not the case, whatever this method returns)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 97.88%. Comparing base (71358ed) to head (e98bb4f).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1941      +/-   ##
==========================================
- Coverage   98.05%   97.88%   -0.18%     
==========================================
  Files          53       53              
  Lines        5511     5539      +28     
==========================================
+ Hits         5404     5422      +18     
- Misses        107      117      +10     

Flag	Coverage Δ
unittests	97.88% <100.00%> (-0.18%)	⬇️

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[pawamoy]

Quick initial review 🙂