Be less restrictive on what characters are disallowed in custom slugs

[acelaya]

Closes #2156

Reduce the strictness on what characters can be used in custom slugs.

At one point, a bug was reported that using ? would create invalid URLs, as the server would then detect anything after the question mark as part of the query string, and never match the short URL.

In order to prevent this, Shlink started to forbid the use of URL-reserved characters in custom slugs, but that broke the experience for users which were using some of those characters.

This PR reduces a bit the strictness, by still forbidding the use of URL-reserved characters that are used to split parts of a URI (gen-delimiters), but allowing other reserved characters (sub-delimiters) like + or &.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 93.71%. Comparing base (509ef66) to head (e80af78).
Report is 3 commits behind head on develop.

Additional details and impacted files

@@            Coverage Diff             @@
##             develop    #2310   +/-   ##
==========================================
  Coverage      93.71%   93.71%           
  Complexity      1680     1680           
==========================================
  Files            276      276           
  Lines           5839     5839           
==========================================
  Hits            5472     5472           
  Misses           367      367           

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