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.

tests/bootstrap/setup.ts

@@ -0,0 +1,4 @@
+import 'mocha';
+import { enableAutoDestroy } from '@vue/test-utils';
+
+enableAutoDestroy(afterEach);