Fixes for create_release_notes.sh

1) Teach it to ignore the sample RELEASE_NOTES_NEXT.txt file
2) Add a proper section for "New Features" heading, instead of
   defaulting everything under the "Upgrade notes" heading.

Note: Will still require manually moving any upgrade notes to the
      right place post-compile.

Signed-off-by: Ben Shum <bshum@biblio.org>
Pre-2.7 Release notes cleanup

1) Fixing up formatting issues with the asciidoc
2) Remove extra words "New feature" from certain notes

Signed-off-by: Ben Shum <bshum@biblio.org>
Compile release notes for Evergreen 2.7.0

Signed-off-by: Ben Shum <bshum@biblio.org>