fix horizontal overlap layout

[tabacitu]

Fixes the issues mentioned in https://github.com/Laravel-Backpack/theme-tabler/issues/59

• adds some extra CSS, for this particular layout, that make it pretty on all CRUD pages;
• removes the urlsUsingOverLapEffect config;
• removes the workaround Pedro added in the ServiceProvider;

After this PR, most pages look pretty good with this layout:

Some addons still need adjusting, though. In the process I've noticed not all pages use the same classes for the Heading. So it's impossible to style all of them using one CSS rule. We should probably use the same syntax for the Heading, across all packages and operation. That way it'll be easy to target, both for our themes and custom themes. So TODOs:

• [x] also whiten the heading on addons (LogManager, BackupManager etc)
• [x] give it another round of testing and see what else doesn't look good in this layout

[tabacitu]

I've tested this all across the board and it works (and looks!) great. Merging it without a double-check, since I'm sure it's good.