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

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

[pombredanne]

@AyanSinhaMahapatra ready to merge