Normalize unix usernames to be under 32char

JupyterHub usernames do not have this restriction,
causing user creation to fail when usernames are too large.

tests/test_normalize.py

@@ -0,0 +1,24 @@
+"""
+Test functions for normalizing various kinds of values
+"""
+from tljh.normalize import generate_system_username
+
+
+def test_generate_username():
+    """
+    Test generating system usernames from hub usernames
+    """
+    usernames = {
+        # Very short
+        'jupyter-test': 'jupyter-test',
+        # Very long
+        'jupyter-aelie9sohjeequ9iemeipuimuoshahz4aitugiuteeg4ohioh5yuiha6aei7te5z': 'jupyter-aelie9sohjeequ9iem-4b726',
+        # 26 characters, just below our cutoff for hashing
+        'jupyter-abcdefghijklmnopq': 'jupyter-abcdefghijklmnopq',
+        # 27 characters, just above our cutoff for hashing
+        'jupyter-abcdefghijklmnopqr': 'jupyter-abcdefghijklmnopqr-e375e',
+
+    }
+    for hub_user, system_user in usernames.items():
+        assert generate_system_username(hub_user) == system_user
+        assert len(system_user) <= 32