Evaluate Bootstrap variables within SASS templates

[jrgriffiniii]

Currently, there are hard-coded references to Bootstrap variables. Please see the following:

https://github.com/samvera/hydra-editor/blob/v6.0.0/app/assets/stylesheets/hydra-editor/multi_value_fields.scss#L3

This assumes that Bootstrap is indeed an upstream dependency. This is not the case (as simple_form does not specify Bootstrap as a dependency: https://github.com/heartcombo/simple_form#installation)