Remove @mentions from PR template and change log. They weren't working.

[PhilRunninger]

Description of Changes

Closes #

---

New Version Info

• Derive a new version number. Increment the:
  • MAJOR version when you make incompatible API changes,
  • MINOR version when you add functionality in a backwards-compatible manner, and
  • PATCH version when you make backwards-compatible bug fixes.
• Update version number in autoload/nerdtree.vim
• Update the ChangeLog, following this format/example:

  MAJOR.MINOR...
  - **.PATCH**: PR Title (Author) #PR Number
  
  ### 5.1...
  - **.1**: Update Changelog and create PR Template (@PhilRunninger) #1007
  - **.0**: Too many changes for one patch...