Add neomutt version of sidebar patch. (closes #3829)
This is the patch from neomutt; branch 'devel/win-sidebar'; commit
c796fa85f9cacefb69b8f7d8545fc9ba71674180 with the following changes:
- move the sample muttrc and vimrc to contrib.
- remove the README.sidebar.
- empty out the PATCHES file.