packs/auto-complete: Switch to company-mode

Signed-off-by: Gergely Nagy <algernon@madhouse-project.org>
packs/lang-clojure: Drop clojure-mode-test

Signed-off-by: Gergely Nagy <algernon@madhouse-project.org>
packs/lang-c: Pull in company-c-headers

Signed-off-by: Gergely Nagy <algernon@madhouse-project.org>
users/algernon[@angthalion]: Various stuff

Rearranged the Gnus setup, and added the underwater-theme (use it by
default, too).

Signed-off-by: Gergely Nagy <algernon@madhouse-project.org>