Add experimental option to reduce whitespace

This reduces the excessive whitespace in the RCE thread page, and fixes
space issues in the thread list on mobile.

Bug: twpowertools:130
Change-Id: Ibd0c666fa7f6a9c89814e529ef7b80541143ada2
diff --git a/src/contentScripts/publicThreadStart.js b/src/contentScripts/publicThreadStart.js
new file mode 100644
index 0000000..d1c4e3c
--- /dev/null
+++ b/src/contentScripts/publicThreadStart.js
@@ -0,0 +1,9 @@
+import {injectStylesheet} from '../common/contentScriptsUtils.js';
+import {isOptionEnabled} from '../common/optionsUtils.js';
+
+isOptionEnabled('uispacing').then(isUiSpacingEnabled => {
+  if (!isUiSpacingEnabled) return;
+
+  injectStylesheet(chrome.runtime.getURL('css/ui_spacing/shared.css'));
+  injectStylesheet(chrome.runtime.getURL('css/ui_spacing/twbasic.css'));
+});