build: check in empty directory gregsite/static

The Django configuration expects a static directory to be present
on the file system, otherwise the tests will output a warning.  This
adds an empty directory using the clever git trick of ignoring
everything within the directory except the .gitignore file itself.

This silences a warning during the test run.

Fixes: GREG-14

.gitignore

@@ -14,4 +14,3 @@ venv/
 
 gregsite/db.sqlite3
 gregsite/settings/local.py
-gregsite/static/

gregsite/static/.gitignore

+*
+!.gitignore