lint: Ban 2-space indentation in our CSS.

Probably most properly we should check for any number of spaces that
isn't 4, but that's a bit more work to do with our linter framework,
and in practice basically every CSS whitespace error we see is 2-space.

tools/lint-all

@@ -311,6 +311,11 @@ def build_custom_checkers(by_lang):
          'description': "Missing whitespace after : in CSS"},
         {'pattern': '[a-z]{',
          'description': "Missing whitespace before '{' in CSS."},
+        {'pattern': '^[ ][ ][a-zA-Z0-9]',
+         'description': "Incorrect 2-space indentation in CSS",
+         'exclude': set(['static/styles/thirdparty-fonts.css']),
+         'strip': '\n',
+        },
         {'pattern': '{\w',
          'description': "Missing whitespace after '{' in CSS (should be newline)."},
         ] + whitespace_rules # type: RuleList