Design Refresh - Update global form fields

Created by: umpox

Global form fields

This PR updates our Bootstrap form fields to match the new design refresh styles:

Link to designs: https://www.figma.com/file/NIsN34NH7lPu04olBzddTw/Design-Refresh-Systemization-source-of-truth?node-id=908%3A4002

Some of these changes, particularly the form color variables were made as part of https://github.com/sourcegraph/sourcegraph/pull/19926

This PR does include changes to our custom-selects, as these are generally simpler elements (we only style the initial field) and we are actively using these across the webapp

This PR does not address:

1. The checkbox and radio refresh styles. We are limited in what we can do to achieve these styles without additional work to create completely custom checkbox/radios. This is de-prioritized for now, see: https://www.figma.com/file/NIsN34NH7lPu04olBzddTw?node-id=908:4001#76759344
2. The toggle refresh styles. We are typically using a separate React component for these, in the interest of keeping this PR small, this will be addressed in a different PR: https://github.com/sourcegraph/sourcegraph/pull/20445