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/static/options/experiments.html b/src/static/options/experiments.html
index 615d5ec..5eddef5 100644
--- a/src/static/options/experiments.html
+++ b/src/static/options/experiments.html
@@ -16,6 +16,7 @@
         <div class="option"><input type="checkbox" id="workflows"> <label for="workflows" data-i18n="workflows"></label> <button id="manage-workflows" data-i18n="workflows_manage"></button></div>
         <div class="option"><input type="checkbox" id="extrainfo"> <label for="extrainfo" data-i18n="extrainfo"></label></div>
         <div class="option"><input type="checkbox" id="nestedreplies"> <label for="nestedreplies" data-i18n="nestedreplies"></label></div>
+        <div class="option"><input type="checkbox" id="uispacing"> <label for="uispacing" data-i18n="uispacing"></label></div>
         <div class="actions"><button id="save" data-i18n="save"></button></div>
       </form>
       <div id="save-indicator"></div>