Connector form: Refactor form controls and improve UX

[flash1293]

The controls part of the connector form needs to be refactored in a bunch of ways:

• Remove leftover styled components
• Get rid of hardcoded pixel values for sizing (instead rely on flexbox layout)
• Restructure component hierarchy and props better to capture all different cases

Right now for the edit case there are two buttons: "Retest source" and "Save changes and test". The former one always tests the last saved version even if there have been changes to the form which is very confusing. This should be simplified to only show a single button - if there are no changes to the form, it says "Retest source", if there are changes to the form it says "Save changes and test" and acts accordingly.

In the source or destination create pages, the submit button is placed inside the card:

The change proposed is to move the button outside of the card to match the current design:

Prototype:
https://www.figma.com/proto/mFVUWYYjDm5zz6FFx8hTP6/FORMS-FIELDS?page-id=1%3A4&node-id=301%3A10567&viewport=606%2C64%2C0.16&scaling=min-zoom&starting-point-node-id=301%3A10567

In the source settings, the save and delete buttons also move. See in design:
https://www.figma.com/file/8aM4gBoW95whyshI5kwVct/01_06_SOURCES?node-id=222%3A2479