Add customization for application policies

**Is your feature request related to a problem? Please describe.**
I customized our local instance with background images and some custom css, which works really good. We have several OAuth Applications with a group policy binding. As expected users without that group get a access denied. Unfortunately, this prompt still uses the default background. Besides changing the background, it would be nice to adjust the error message or add additional text.

**Describe the solution you'd like**
Add a field to the policy binding to set a custom denial prompt (which could be defined within the prompt menu).

**Describe alternatives you've considered**
I removed the policy binding from the OAuth application and instead created a new flow with a custom static prompt, which contains my default message. As a validator I added a custom policy that checks for the group. It does work, but even users with correct groups have to view that prompt and click continue. That is a bit inconvenient.

**Additional context**
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