Autumn Cleaning - Cleanup in Database Error Page

Removing isDevMode since this concept is handled in Startup.cs.
Keeping UseDatabaseErrorPage behavior consistent with general ASP.NET UseErrorPage:
* Parameterless overload results in everything being shown.
* Adding DatabaseErrorPageOptions.ShowAll pattern to allow UseDatabaseErrorPage(DatabaseErrorPageOptions.ShowAll)

Updating UseDatabaseErrorPage to also register the Migrations Endpoint when it's going to be needed
No problem, we'll still honor that in the error page and not display the button.