Add GitHub issue templates for feature requests and UX reports

.github/ISSUE_TEMPLATE/feature-request.md

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+---
+name: Feature request
+about: Suggest an idea for zcashd.
+title: ''
+labels: 'use case'
+assignees: ''
+
+---
+
+## Is your feature request related to a problem? Please describe.
+A clear and concise description of what the problem is. Example: I'm always
+frustrated when [...]
+
+## Describe the solution you'd like
+A clear and concise description of what you want to happen.
+
+## 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.

.github/ISSUE_TEMPLATE/ux-report.md

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+---
+name: UX report
+about: Was zcashd hard to use? It's not you, it's us. We want to hear about it.
+title: 'UX: '
+labels: 'usability'
+assignees: ''
+
+---
+
+<!-- Did zcashd not do what you expected?
+Was it hard to figure out how to do something?
+Could an error message be more helpful?
+It's not you, it's us. We want to hear about it. -->
+
+## What were you trying to do
+
+## What happened