Upgrade to the new Github issue templates (#1338)

Author: odow

.github/ISSUE_TEMPLATE.md

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+---
+name: Bug report
+about: Help us track down bugs in JuMP
+
+---
+
+Welcome to JuMP!
+
+Please read the following before posting a new bug report:
+
+- If you have a question or are unsure if the behavior you're experiencing is a bug, please search or post to our Discourse site: https://discourse.julialang.org/c/domain/opt. Questions posted to Discourse have broader visibility and are likely to be answered more quickly than issues filed here.
+
+- If you're experiencing a bug that is solver-specific (e.g., it only happens when you use Gurobi), the issue is best raised at the relevant solver-wrapper repository (e.g., Gurobi.jl). If you are unsure, you should raise the problem on Discourse first.
+
+- If you are reasonably confident your issue is a bug in JuMP, this is the right place. Be sure to include as much relevant information as possible, including a minimal reproducible example. See https://help.github.com/articles/basic-writing-and-formatting-syntax/ for background on how to format text and code on GitHub issues.
+
+Thanks for contributing to JuMP!

.github/ISSUE_TEMPLATE/bug_report.md

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+---
+name: Feature request
+about: Suggest an idea for JuMP
+
+---
+
+Welcome to JuMP! You can use this Github issue to suggest a new feature for JuMP.
+
+**Is your feature request related to a problem? Please describe.**
+A clear and concise description of what the problem is. Ex. I'm always frustrated when [...]
+
+**Describe the solution you'd like**
+A clear and concise description of what you want to happen.
+
+**Describe alternatives you've considered**
+A clear and concise description of any alternative solutions or features you've considered.
+
+**Additional context**
+Add any other context or screenshots about the feature request here.
+
+Thanks for contributing to JuMP!