Search filters should display on code reference landing page

[siobhan]

Currently we just have a simple search bar for the code reference, which is nice, but it would be better if there were search filters on the front page that people could use to filter prior to search.

I suggest the following filters:

• function
• action
• filter
• method
• class

[coffee2code]

In 744:

Code Reference: add post_type search filter checkboxes to search form (all checked by default). Fixes #552