refactor(frontend): Button with intent and size replaces the text buttons

[Kadrian]

Design-system prep, step 6: the Button

Stacked on #4005. ui-components/Button replaces PrimaryButton, TransparentButton, DestroyButton and SelectFileButton: one component, two axes, fixed heights. IconButton follows in the next step.

<Button intent="primary" size="md" onPress={save}><Icon icon={faCheck} />Save</Button>
<Button intent="tertiary" aria-label="Delete"><Icon icon={faTrash} /></Button>

• intent: primary (filled), secondary (outlined, the default), tertiary (ghost with hover background), danger (outlined red, fills on hover), link (reads as a text link, no box, no padding; the dropzones' import buttons)
• size: sm 24, md 30 (the default, 88 of the app's 119 buttons), lg 36 px, each a fixed height with its own text size and icon gap, so text buttons and icon buttons line up; a button whose only children are icons is a square of the same height
• pressed state is aria-pressed, which colors the button; the active flag is gone
• react-aria's API throughout: onPress (press events don't bubble, the one stopPropagation went), isDisabled, autoFocus, excludeFromTabOrder
• Button stories: intents, all sizes with text, icon plus text and icon-only, pressed state

[awildturtok]

Danke auch für die Tests!