Add issue templates

.github/ISSUE_TEMPLATE/bug-report.md

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+---
+name: Bug Report
+about: Report a bug encountered while using Yorkie
+labels: bug 🐞
+
+---
+
+<!-- Please use this template while reporting a bug and provide as much info as possible. Not doing so may result in your bug not being addressed in a timely manner. Thanks!
+-->
+
+
+**What happened**:
+
+**What you expected to happen**:
+
+**How to reproduce it (as minimally and precisely as possible)**:
+
+**Anything else we need to know?**:
+
+**Environment**:
+- Android Studio version:
+- Yorkie version (use `yorkie version`):
+- Yorkie Android SDK version:

.github/ISSUE_TEMPLATE/common-issue.md

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+---
+name: Common Issue
+about: A common issue with the Yorkie project
+labels:
+
+---
+<!-- Please only use this template for submitting common issues -->
+
+**Description**:
+
+**Why**:

.github/ISSUE_TEMPLATE/config.yml

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+blank_issues_enabled: false

.github/ISSUE_TEMPLATE/feature-request.md

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+---
+name: Enhancement Request
+about: Suggest an enhancement to the Yorkie project
+labels: enhancement 🌟
+
+---
+<!-- Please only use this template for submitting enhancement requests -->
+
+**What would you like to be added**:
+
+**Why is this needed**: