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.

static/styles/overlay.css

@@ -85,8 +85,8 @@
 }
 
 #overlay .image-description {
-  /* approx width of screen minus action buttons on the side. */
-  width: calc(100% - 290px);
+    /* approx width of screen minus action buttons on the side. */
+    width: calc(100% - 290px);
 }
 
 #overlay .image-description .title {
@@ -108,7 +108,7 @@
 }
 
 .image-actions a.button {
-  text-decoration: none;
+    text-decoration: none;
 }
 
 .button .download {

static/styles/portico.css

@@ -945,7 +945,7 @@ a.bottom-signup-button {
 }
 
 .center-block .submit-button-box .controls {
-  margin: 0px;
+    margin: 0px;
 }
 
 .center-block {
@@ -975,7 +975,7 @@ a.bottom-signup-button {
 }
 
 .center-block #send_confirm input[type=text] {
-  margin-top: 20px;
+    margin-top: 20px;
 }
 
 .center-block .button {

static/styles/settings.css

@@ -8,7 +8,7 @@ label {
 }
 
 .display-none {
-  display: none;
+    display: none;
 }
 
 .inline-block {
@@ -171,7 +171,7 @@ input[type=checkbox].inline-block {
 }
 
 .remove-alert-word {
-  margin-top: 1px;
+    margin-top: 1px;
 }
 
 @media (max-width: 480px) {

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