[TASK] Unify button styles

Unifies buttons over the backend:

+ <button>
+ <input type="submit">
+ <input type="reset">
+ <a class="t3-button">

Extensions do not need to bring own styling for buttons.
Also adds padding and margin to input, buttons.

Change-Id: Ie1ab255354a222acfbac9b3260297ff345b6feed
Resolves: #55319
Releases: 6.2
Reviewed-on: https://review.typo3.org/27048
Reviewed-by: Wouter Wolters
Tested-by: Wouter Wolters
Reviewed-by: Marcin Sągol
Reviewed-by: Markus Klein
Tested-by: Markus Klein