Improve copyright detection

[pombredanne]

This PR improves many areas of copyright detection for correctness and false positive.
In particular:

• The way we strip text from markup tags has been entirely reworked, replacing an obscure and untested regex with a simpler splitter regex and a set of known tags to strip.
• Handling of various tag-like strings such as <https://some.url.com> and emails has been modified and improved
• A large number of issues and false positive have been fixed, in particular by re-scanning Linux.
• Many new tests have been added along the way
• The code to select candidate lines and prepare text lines has been streamlined and simplified

Tasks

• Reviewed contribution guidelines
• PR is descriptively titled 📑 and links the original issue above 🔗
• Tests pass -- look for a green checkbox ✔️ a few minutes after opening your PR
  Run tests locally to check for errors.
• Commits are in uniquely-named feature branch and has no merge conflicts 📁
• Updated documentation pages (if applicable)
• Updated CHANGELOG.rst (if applicable)

[pombredanne]

@AyanSinhaMahapatra ready to merge

[AyanSinhaMahapatra]

Thanks @pombredanne ! Merging this