Fix design of "Conflict when hiding content item" modal, based on standards defined in UI Guidelines:

Modals -> https://doc.ezplatform.com/en/latest/guidelines/components/modals/

• Modal body: Add standard padding for this kind of modal;
• Modal footer: remove float: right;, center set of buttons.

Buttons -> https://doc.ezplatform.com/en/latest/guidelines/components/buttons/#set-of-two-buttons

• Set of two buttons, add standard margin between them;
• Set bold font-weight for preferred primary action button, as indicated in UI Guidelines.