Make all of Magit configurations in use-package.

This was causing startup troubles-- I believe this is because
use-package defers loading, whereas the function was expecting some of
the post-load magit stuff (specifically the keymap) to be there.
Convert diminish and calls to use-package.

Using require would prevent Emacs from booting my
configuration. Moreover, using the :diminish marker within use-package
was not diminishing reliably. Not sure if this is on my end,
the end of the packages, or of use-package. At any rate, using
use-package to call diminish (at the end of the configuration, as
recommended by Will Mengarini) and then making specific commands on
:init works for now. This may prove unsustainable, however, if I want to
diminish minor modes that are not always present (unlike smartparens and
company-mode).