Chore: Refactor: Mobile: Rename CustomButton to IconButton

[personalizedrefrigerator]

Summary

Previously, CustomButton was only used to display icon buttons. This pull request renames CustomButton to IconButton and moves the icon logic into CustomButton.

Note: This refactoring change was originally made as a part of #10465, before adding a new custom button component wrapping React Native Paper's <Button>.

Notes

• To simplify icon-rendering logic, some toolbar icons (attach, hide keyboard, search, horizontal dots) were switched from Material icons to Material Community icons:
  
  Notice that the "attach" icon is now slightly thinner.

Testing plan

1. Open the note editor. Long-press on "find and replace", "link", and "more". Verify that tooltips are shown for each.
2. Click the "⋯" button to show more toolbar actions.
3. Attach a file.
4. Undo.
5. Verify that both the undo and redo buttons have the expected icons.
6. Close the note editor.
7. Close the note viewer.
8. Select multiple notes.
9. Verify that all toolbar buttons have icons
10. Delete at least one note.
11. Navigate to the trash folder and select the note.
12. Verify that the toolbar buttons are present (and not the "missing icon" icon).
13. Restore the note.
14. Open search.
15. Close search.
16. Install at least one plugin that adds a panel.
17. Verify that the plugin panel toggle button has a puzzle piece icon.

This has been tested successfully on an iOS 17 simulator.