Various CSS improvements (#514)

* Replace flexbox grid with CSS grid

* Update new and edit forms

* Update settings views

* Update auth views

* Fix margin in menu

* Remove unused Spectre modules

* Simplify navbar

* Reuse CSS variables

* Fix grid gap on small screen sizes

* Simplify grid system

* Improve section headers

* Restructure SASS files

* Cleanup base styles

* Update test

bookmarks/styles/bookmark-form.scss

@@ -0,0 +1,50 @@
+.bookmarks-form {
+
+  .btn.form-icon {
+    padding: 0;
+    width: 20px;
+    height: 20px;
+    visibility: hidden;
+    color: $gray-color;
+
+    &:focus,
+    &:hover,
+    &:active,
+    &.active {
+      color: $gray-color-dark;
+    }
+
+    > svg {
+      width: 20px;
+      height: 20px;
+    }
+  }
+
+  .has-icon-right > input, .has-icon-right > textarea {
+    padding-right: 30px;
+  }
+
+  .has-icon-right > input:placeholder-shown ~ .btn.form-icon,
+  .has-icon-right > textarea:placeholder-shown ~ .btn.form-icon {
+    visibility: visible;
+  }
+
+  .form-icon.loading {
+    visibility: hidden;
+  }
+
+  .form-input-hint.bookmark-exists {
+    display: none;
+    color: $warning-color;
+
+    a {
+      color: $warning-color;
+      text-decoration: underline;
+      font-weight: bold;
+    }
+  }
+
+  details.notes textarea {
+    box-sizing: border-box;
+  }
+}