Rename page_heading_actions variable (#12799)

Current name - page_heading_actions is long and hard to remember,
this commit shortens it to just heading_actions, which is way more
easier to remember and write, it's logical too.

app/views/filters/index.html.haml

@@ -1,7 +1,7 @@
 - content_for :page_title do
   = t('filters.index.title')
 
-- content_for :page_heading_actions do
+- content_for :heading_actions do
   = link_to t('filters.new.title'), new_filter_path, class: 'button'
 
 - if @filters.count == 0