Use absolute paths for SCSS imports

A recent update in [GitLab UI] and setting the [config.assets.paths] to
include `node_modules` for the resolution of SCSS imports, allows us to
have much nicer imports which don't rely on brittle relative paths.

This change also forwards @gitlab/ui to the newest version

[GitLab UI]: https://gitlab.com/gitlab-org/gitlab-ui/merge_requests/602
[config.assets.paths]: https://gitlab.com/gitlab-org/gitlab-ce/blob/d9026d0ffa0/config/application.rb#L169

app/assets/stylesheets/application.scss

@@ -11,10 +11,10 @@
 // like a table or typography then make changes in the framework/ directory.
 // If you need to add unique style that should affect only one page - use pages/
 // directory.
-@import "../../../node_modules/at.js/dist/css/jquery.atwho";
-@import "../../../node_modules/pikaday/scss/pikaday";
-@import "../../../node_modules/dropzone/dist/basic";
-@import "../../../node_modules/select2/select2";
+@import "at.js/dist/css/jquery.atwho";
+@import "pikaday/scss/pikaday";
+@import "dropzone/dist/basic";
+@import "select2/select2";
 
 // GitLab UI framework
 @import "framework";

app/assets/stylesheets/csslab.scss

-@import "../../../node_modules/@gitlab/csslab/dist/css/csslab-slim";
+@import "@gitlab/csslab/dist/css/csslab-slim";

app/assets/stylesheets/errors.scss

@@ -2,12 +2,12 @@
  * This is a minimal stylesheet, meant to be used for error pages.
  */
 @import 'framework/variables';
-@import '../../../node_modules/bootstrap/scss/functions';
-@import '../../../node_modules/bootstrap/scss/variables';
-@import '../../../node_modules/bootstrap/scss/mixins';
-@import '../../../node_modules/bootstrap/scss/reboot';
-@import '../../../node_modules/bootstrap/scss/buttons';
-@import '../../../node_modules/bootstrap/scss/forms';
+@import 'bootstrap/scss/functions';
+@import 'bootstrap/scss/variables';
+@import 'bootstrap/scss/mixins';
+@import 'bootstrap/scss/reboot';
+@import 'bootstrap/scss/buttons';
+@import 'bootstrap/scss/forms';
 
 $body-color: #666;
 $header-color: #456;

app/assets/stylesheets/framework.scss

@@ -2,7 +2,7 @@
 @import 'framework/variables_overrides';
 @import 'framework/mixins';
 
-@import '../../../node_modules/@gitlab/ui/scss/gitlab_ui';
+@import '@gitlab/ui/scss/gitlab_ui';
 
 @import 'bootstrap_migration';
 @import 'framework/layout';

package.json

@@ -38,7 +38,7 @@
     "@babel/preset-env": "^7.4.4",
     "@gitlab/csslab": "^1.9.0",
     "@gitlab/svgs": "^1.65.0",
-    "@gitlab/ui": "^4.3.0",
+    "@gitlab/ui": "^5.1.0",
     "apollo-cache-inmemory": "^1.5.1",
     "apollo-client": "^2.5.1",
     "apollo-link": "^1.2.11",