Add flattenthreads experiment

This experiment allows users to flatten the replies in threads, so they
are shown linearly in a chronological way instead of nested.

When the option is enabled, a switch is added to the thread page which
lets the user switch between flattening replies and not flattening them.

Some UI is still missing (see the design document[1]).

[1]: https://docs.google.com/document/d/1P-HanTHxaOFF_FHh0uSv0GIhG1dxWTJTGoT6VPjjvY0/edit

Bug: twpowertools:153
Change-Id: I43f94442cadc12b752700f0e8d974522be621d3e
diff --git a/src/static/options/experiments.html b/src/static/options/experiments.html
index 615d5ec..18fa34d 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="flattenthreads"> <label for="flattenthreads">Flatten threads (feature name TBD)</label></div>
         <div class="actions"><button id="save" data-i18n="save"></button></div>
       </form>
       <div id="save-indicator"></div>