Change how themes, fonts & modelines are loaded

WARNING: THIS IS A BREAKING CHANGE FOR THEME/FONT/NLINUM CUSTOMIZATIONS.

This change was motivated by the need to decouple theme and font loading
from the ui/doom module.

Now, it is doom-core's purview. Theme and fonts are loaded after
initfiles are read (attached to the doom-init-ui-hook hook), giving
other modules (especially private ones) a chance to change the theme or
fonts.

+ Refactor core-ui.el
+ New init hook: doom-init-ui-hook
+ Decouple theme/font loading from ui/doom
+ Load modelines are doom-init-ui-hook
+ New theme/font variables (replaces old ui/doom variables)
  + doom-theme
  + doom-font
  + doom-variable-pitch-font
  + doom-unicode-font
+ Change nlinum variables
  + doom-line-number-lpad
  + doom-line-number-rpad
  + doom-line-number-pad-char

Addresses #117

modules/private/hlissner/init.el

@@ -14,7 +14,7 @@
          +doom-font (font-spec :family "Fira Mono" :size 10)
          +doom-variable-pitch-font (font-spec :family "Fira Sans" :size 10)
          +doom-unicode-font (font-spec :family "DejaVu Sans Mono" :size 10)
-         doom-ui-nlinum-lpad 3))
+         doom-line-number-lpad 3))
   ("halimede"
    (setq +doom-modeline-height 27))
   ;; ("nereid")