docs: consistent spacing around headings

Start out with 2 empty lines before/after for top level, decrease for deeper
levels.

"Users groups" is grammatically incorrect English - rename to "user groups"

- and same story with "repositories groups"

This will only change the user facing texts. Names of files, modules, functions
and variables should be changed too.

#518 multiple issues patterns
- added logging
- some code cleanup
- added example of wiki pattern to .ini file
- updated contributors and changelog

fixed #570 explicit users group permissions can overwrite owner permissions
- added test for that case

fixes #550 mercurial repositories comparision failed when origin repo had
additional not-common changesets + added regresion test for this

sync alternative gravatar options to other .ini files
- changelog update

RhodeCode now has a option to explicitly set forking permissions. ref #508
- changed the way permissons on users groups behave. Now explicit set on user
is more important than permission set on users group

Fixed validators for remote repos
- use proper httppeer repo for mercurial 2.3
- validate git repos for remote auth
- docs updates

fixes #481 rhodecode emails are sent without Date header
- small code cleanup in mailing

created rhodecode-api binary script for working with api via cli
- created docs
- moved the backup script to bin folder

#469 added --update-only option to whoosh to re-index only given list
of repos in index