refactor(toast): Replace vue-toasted with BootstrapVue's Toast plugin

Drops vue-toasted in favor of BootstrapVue's built-in toast component.

BREAKING CHANGE: This replaces the internal toasts library.

BootstrapVue's built-in toasts are now used instead of `vue-toasted`.

As a result, the following changes should be made when upgrading:

* The following options have been renamed:
    * `duration` now is `autoHideDelay`
    * `className` now is `toasClass`.
* The following options are not supported anymore:
    * `fullWidth`
    * `fitToScreen`
    * `containerClass`
    * `iconPack`
    * `Icon`
    * `theme`
    * `closeOnSwipe`
    * `keepOnHover`
    * `singleton`
    * `type` (note that while this option was previously supported, it
      didn't have any effect visually and can thus be removed altogether)
* It is not possible to render HTML in toasts' body anymore, make sure
  to only pass plain text to the `.show()` method.
* To adhere with the design system, the `position` option has also been
  removed. Toasts should only appear in the bottom-left corner.
* The callbacks have been backported:
    * `onComplete` should work as usual.
    * `onClick`'s second argument is an object with the following
      properties:
      * `id` is the BootstrapVue toast's ID.
      * `hide` is a method that can be called to hide the toast. It
        replaces `goAway(0)`.