Convert notmuch-vim to Vim addon policy 2.0

Convert notmuch-vim to Vim addon policy 2.0

The new Vim addon policy relies on Vim's native package functionality,
rather than the Debian-specific vim-addons tool.  This allows the addon
to be immediately available when the package is installed, rather than
requiring extra setup by the user.

* Install notmuch-vim to its own /usr/share/notmuch-vim directory, so
  the files are self-contained.
* Remove vim-addon-manager from notmuch-vim Depends
* Add ${vim-addon:Depends} to notmuch-vim Depends to ensure it has the
  required relationships on Vim/Neovim.
* Build-Depend on dh-sequence-vim-addon to automatically setup the
  required symlinks for Vim and Neovim to see the addon.

Signed-off-by: James McCoy <jamessan@debian.org>