Update issue templates

Author: beyang

.github/ISSUE_TEMPLATE/stop--do-not-file-issues-here.md

@@ -0,0 +1,12 @@
+---
+name: 'STOP: Do not file issues here'
+about: File issues in github.com/sourcegraph/sourcegraph
+title: ''
+labels: ''
+assignees: ''
+
+---
+
+Issues filed in this repository will generally be ignored. This issue tracker exists only so bots can open issues when necessary.
+
+File issues in the Sourcegraph main repository: github.com/sourcegraph/sourcegraph.