Refactor button classes for conditional margin

[eduardomozart]

Updated button classes to conditionally apply margin-top based on params.

Checklist before requesting a review

Please delete options that are not relevant.

• I have read the CONTRIBUTING document.
• I have performed a self-review of my code.
• I have added tests that prove my fix is effective or that my feature works.
• This change requires a documentation update.

Description

• It fixes # (issue number, if applicable)
• Here is a brief description of what this PR does:

Add a margin-top for action buttons on modals. Tested in:

• Creating a new external event on Planning (/front/planning.php)
• Creating a new Plug in PDU (/front/pdu.form.php).

Screenshots (if appropriate):

Before:

After: