Update issue templates (#471)

Use two different templates instead of one: one for bugs, one for feature requests.

.github/ISSUE_TEMPLATE/bug_report.md

@@ -0,0 +1,18 @@
+---
+name: Bug report
+about: Create a report to help us improve
+title: ''
+labels: 'C: Bug'
+assignees: ''
+
+---
+
+<!-- Describe your bug, explaining how to reproduce it, and what was expected -->
+
+
+
+<!-- Please fill the following information when relevant (or remove them if not) -->
+
+- **Plume version:** You can find it in the footer of your instance. If you know the exact commit, please also add it.
+- **Operating system:**
+- **Web Browser:**

.github/ISSUE_TEMPLATE/feature_request.md

@@ -0,0 +1,20 @@
+---
+name: Feature request
+about: Suggest an idea for this project
+title: ''
+labels: ''
+assignees: ''
+
+---
+
+**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.