fix(dropdown): Align dropdown caret to right when block prop is present

Aligns the gl-new-dropdown caret to the right when the component has the block prop present.

We currently don't have access the the button element in the dropdown. So I've wrote some more code in the mounted hook for applying classes based on props. I opened an MR to get around this against bootstrap-vue, it's merged but we can't use it until we upgrade BV to 2.9.0.

https://github.com/bootstrap-vue/bootstrap-vue/pull/5014

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After