add cross references for all Custom groups from git-modes
drop support for magit-log-edit.el
remove some obsolete variables
run-hook-wrapped{,-1}: switch positions of definitions

I try to get people to follow the "entry-point before gory details"
style, and so it is important not to break that on the very first
opportunity.
magit-grep: remove alias for vc-git-grep (or lgrep)
magit-version: define variable right before function
magit-version: use error, not user-error, on failure

Because it ain't no user error.  Also we take care of the "we cannot
have no error here" case by providing a NOERROR argument.
magit-font-lock-keywords: remove magit-define-level-shower

That macro was recently removed.
magit.el: remove unnecessary declarations
AUTHORS.md: update list of contributors
replace version 2.0.0 with 2.1.0

There will be no 2.0.0 release, as we have switched to a version scheme
inspired by that of Emacs.