Properly wrap Bootstrap modal in modal.vue

See this discussion for more context.

We're currently only using Bootstrap's CSS for our popup modal, and showing/hiding it with a simple v-if and controlled by a passed prop. However, Bootstrap's JS includes more robust event handling, normalizes some styling issues, adds some light animation, and exposes a public API we can hook into.

Given that this component is meant to be shared, I think it would make sense to wrap the Bootstrap modal, taking advantage of what it has to offer but with Vue's declarative style.