feat: Screwdriver style mixin for Bootstrap buttons

[minghay]

Context

Create a style mixin for bootstrap buttons so that we can incrementally adopt a consistent standard on button styles on the new UI.

Objective

Setting the primary and outline button styles based on bootstrap ember buttons (i.e., BsButton components).

Primary button:

Primary button hover state:

Primary button as outline style (note that the button background color is actually transparent hence the slight grey color in the attached screenshot):

Primary button as outline hover state:

References

screwdriver-cd/screwdriver#3200

License

I confirm that this contribution is made under the terms of the license found in the root directory of this repository's source tree and that I have the authority necessary to make this contribution on behalf of its copyright owner.