refactor(init): Refactor package installation process

Refactors package installation to be slightly more sane, for example
package-refresh-contents will only be called if packages are missing.

Removes some other old cruft, too, and paves way for a slightly
different initialisation process.

init/theme.el

@@ -0,0 +1,6 @@
+;;; theme.el --- Editor theming. -*- lexical-binding: t; -*-
+
+;; Configure smart mode line
+(sml/setup)
+
+(provide 'theme)