Reorder and align boards search bar buttons [RUN AS-IF-FOSS]

What does this MR do?

1. Group by button next to Show labels, so buttons are all together
2. Align label text for toggles and dropdowns
3. Vertically align all buttons
4. Convert Group by and fullscreen buttons to gl-buttons
5. Add flex + center all buttons
6. (bonus) remove vertical scrollbar from body on boards page

Screenshots

Before

Alignment everywhere, bunch of non-gl-buttons, some things the wrong height.

After

All gl-buttons, all centered and evenly spaced.

FOSS:

EE:

EE with new_board_lists feature enabled:

Closes #291985 (closed)

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Separation of EE specific content