Implement custom lightweight modal #230

Introduce a brand new lightweight and efficient modal component. It is
designed to be visually similar to the previous one to not introduce a
change in feel of the application in a patch release, but behind the
scenes it features:

- Enhanced application speed and reduced bundle size.
- New flexbox-driven layout, eliminating JS calculations.
- Composition API ready for Vue 3.0 #230.

Other changes:

- Adopt idiomatic Vue via `v-modal` binding.
- Add unit tests for both the modal and dialog.
- Remove `vue-js-modal` dependency in favor of the new implementation.
- Adjust modal shadow color to better match theme.
- Add `@vue/test-utils` for unit testing.

src/presentation/assets/styles/_mixins.scss

@@ -27,3 +27,21 @@
     */
     -webkit-tap-highlight-color: transparent;
 }
+
+@mixin fade-slide-transition($name, $duration, $offset-upward: null) {
+    .#{$name}-enter-active,
+    .#{$name}-leave-active {
+        transition: all $duration;
+    }
+
+    .#{$name}-leave-active,
+    .#{$name}-enter, // Vue 2.X compatibility
+    .#{$name}-enter-from // Vue 3.X compatibility
+    {
+        opacity: 0;
+
+        @if $offset-upward {
+            transform: translateY($offset-upward);
+        }
+    }
+}