Improve issue handling [skip ci]

.github/ISSUE_TEMPLATE.md

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+### Prerequisites
+
+- [ ] I have written a descriptive issue title
+- [ ] I have verified that I am running the latest version of ImageSharp
+- [ ] I have verified if the problem exist in both `DEBUG` and `RELEASE` mode
+- [ ] I have searched [open](https://github.com/JimBobSquarePants/ImageSharp/issuess) and [closed](https://github.com/JimBobSquarePants/ImageSharp/issues?q=is%3Aissue+is%3Aclosed) issues to ensure it has not already been reported
+
+### Description
+<!-- A description of the bug or feature -->
+
+### Steps to Reproduce
+<!-- List of steps, sample code, failing test or link to a project that reproduces the behavior -->
+
+### System Configuration
+<!-- Tell us about the environment where you are experiencing the bug -->
+
+- ImageSharp version:
+- Other ImageSharp packages and versions:
+- Environment (Operating system, version and so on):
+- .NET Framework version:
+- Additional information:
+
+<!-- Thanks for reporting the issue to ImageSharp! -->

.github/PULL_REQUEST_TEMPLATE.md

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+### Prerequisites
+
+- [ ] I have written a descriptive pull-request title
+- [ ] I have verified that there are no overlapping [pull-requests](https://github.com/JimBobSquarePants/ImageSharp/pulls) open
+- [ ] I have verified that I am following matches the existing coding patterns and practise as demonstrated in the repository. These follow strict Stylecop rules :cop:.
+- [ ] I have provided test coverage for my change (where applicable)
+
+### Description
+<!-- A description of the changes proposed in the pull-request -->
+
+<!-- Thanks for contributing to ImageSharp! -->