Add buttons to hide/show left and right panel columns

After some testing on snapshot, here are comments/improvements that were found:
1. `when I hide a panel and then switch to "mobile mode" by making the window smaller, the panel stays hidden but there is no more button to get it back?`
2. `the padding we added to the make the panels symmetric again should be removed for "mobile mode" as they add assymetric padding.`
3. `in standalone edit mode (like when creating a page), the panel widths are applied but it isn't possible to resize the panel`

1. should at least be fixed by a reset of the page.
2. Should be a small style fix. Probably didn't catch this because it was added after the most extensive test session was done.
3. interaction of the system with the width?