init.el: add my-package-reset

This will reset all the packages to a known manifest. I've made it a
function rather than call it on start-up as it will kill installed
packages not in the manifest.
init.el: clean-up package list

* rm magithub
* add yasnippet
my-email.el: moved my directories around a little
init.el: add markdown-mode and mediawiki mode to pkg list
init.el: clean-up mail-mode/message-mode bindings

I'll use mail-mode for server-edit related things and message-mode from
within emacs itself (like mu4e). Hopefully the keybinding doesn't
corrupt the whole mode now.
my-email: use the new header flow option.
my-email: add bookmark for mail I sent
my-email: initial my-insert-pull-request

This is a little ugly but functional. It could be a lot smarter though
and I suspect it could live somewhere else.