Merge carbon-project-tools back into carbon-lang (#203)

As proposed at https://forums.carbon-lang.dev/t/merged-repo-structure/174:

- carbon-lang:/src/jekyll -> carbon-lang:/website/jekyll
- carbon-lang:/src/scripts -> carbon-lang:/proposals/scripts
- carbon-project-tools:/firebase -> carbon-lang:/website/firebase
- carbon-project-tools:/github -> carbon-lang:/github

github/README.md

@@ -0,0 +1,17 @@
+# GitHub
+
+<!--
+Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+Exceptions. See /LICENSE for license information.
+SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+-->
+
+Scripts for use with GitHub.
+
+See individual scripts for more details.
+
+Please use `pytest` for testing:
+
+```
+$ pip install pytest
+```