File dialog is too small for video picker

The file picker currently has a [hardcoded height of 25rem](https://github.com/hypothesis/lms/blob/ddf4cc6c10ef01a373eb22fc2c7adc4b907f5509/lms/static/scripts/frontend_apps/components/LMSFilePicker.tsx#L408). This is too small for when we're displaying videos in the picker. Here there is only room for two and a half items:

The breadcrumbs and dialog title use up a lot of vertical space here, so we could free up some by re-arranging / altering those. However we should also just alter the height to use more vertical space if we have it.