chore: enable Dependabot and add pull request template (#77)

Author: unknwon

.github/dependabot.yml

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+# Docs: https://git.io/JCUAY
+version: 2
+updates:
+  - package-ecosystem: "gomod"
+    directory: "/"
+    schedule:
+      interval: "weekly"
+    reviewers:
+      - "gogs/core"
+    commit-message:
+      prefix: "mod:"

.github/pull_request_template.md

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+### Describe the pull request
+
+A clear and concise description of what the pull request is about, i.e. what problem should be fixed?
+
+Link to the issue: <!-- paste the issue link here, or put "n/a" if not applicable -->
+
+### Checklist
+
+- [ ] I agree to follow the [Code of Conduct](https://go.dev/conduct) by submitting this pull request.
+- [ ] I have read and acknowledge the [Contributing guide](https://github.com/gogs/gogs/blob/main/.github/CONTRIBUTING.md).
+- [ ] I have added test cases to cover the new code.