Mostly just a revert of the commit that removed this functionality, with a few changes:
* Perform proper JavaScript encoding in the script part of the database error page Views/BaseView.cs
* Use the builder pattern in the UseXyz extension methods on IApplicationBuilder (per #184). Also applying this to DatabaseErrorPageOptions to keep things consistent.
* Fixing a few tests that were getting the context from DI but putting it in a using block so that it got disposed (rather than letting DI handle disposing).
- Removed `SetDefaultVisibility` method from `(Database)ErrorPageOptions`
- `(Database)ErrorPageOptions` properties are now auto-properties since there is no default visibility anymore
- Removed `isDevMode` parameter from `ErrorPageMiddleware` ctor
- Removed comments from `ErroPageExtensions.UseErrorPage()`
- Updated Entity tests
#130
Rowan Miller
Henk Mollema
N. Taylor Mullen