Feat/new strategy variants tab (#5649)

This PR sets up the variants tab for the new strategy configuration.
Also some minor adjustments to the new form:

* Change where padding is controlled to allow us to have more granular
control over how the buttons width and border should look and have the
tabs have full-width borders across the form
* Move the buttons to be absolutely positioned at the bottom of the page
* Move where we display banners to avoid clutter

<img width="1284" alt="Skjermbilde 2023-12-14 kl 21 17 53"
src="https://github.com/Unleash/unleash/assets/16081982/45e6a364-e4aa-47ac-b420-f3be9b39a15e">