Turn on declaration-no-important stylelint rule

This rule was accidentally missed in the initial config for
stylelint-config-gds. This applies disabling of this rule in the
situations where we make an exception and want important rules.

.stylelintrc.yml

@@ -4,7 +4,6 @@ ignoreFiles: src/govuk/vendor/**/*
 plugins:
   - stylelint-order
 rules:
-  declaration-no-important: ~
   # govuk-frontend has a specific ordering pattern that should be applied to
   # rules
   #

src/govuk/overrides/_display.scss

@@ -4,6 +4,7 @@
 
 @import "../base";
 
+// stylelint-disable declaration-no-important
 @include govuk-exports("govuk/overrides/display") {
   .govuk-\!-display-inline {
     display: inline !important;

src/govuk/overrides/_width.scss

@@ -4,6 +4,7 @@
 
 @import "../base";
 
+// stylelint-disable declaration-no-important
 @include govuk-exports("govuk/overrides/width") {
   .govuk-\!-width-full {
     width: 100% !important;