Override Sphinx integrated search with our indexes

[agjohnson]

The Sphinx search "works", but our search returns more relevant results. This work will include an integrated front end interface for search results on generated documentation which pulls from our existing indexes.

Implementing this feature might be difficult on a wider basis, as themes all load their local _static/searchtools.js. The easiest place to start would be by reimplementing searchtools.js with a similar interface, or extending the existing implementation, and overriding this in our local template overrides path. This would only affect projects using our theme, or derivatives of, for now.

The current searchtools.js implementation uses it's own search index data and performs its own text normalization and result scoring. An overridden implementation would rely on an elastic search backend, with supplemental information from the existing implementation. The current index has information on domain objects, which aren't surfaced in the same way in the search index. Results could be merged into the same search results tree, for display output. Features like highlighting in search results and the current search UI elements will be maintained.

• Provide basic override of search tools javascript from Sphinx
• Test with other themes
• Test sub/super project display on listings
• Explore merging domain objects -- can we detect duplicates easily in search results, or are search results enough?
• Refactor to use search.lib
• Move searchtools override to RTD sphinx ext
• Fix linking to projects, add canonical url mapping for projects

This relies on readthedocs/readthedocs-sphinx-ext#16

[agjohnson]

This is now unblocked. I created a sphinx_rtd_ext release branch of 0.6-alpha for staging some of our changes to moving changes into the ext. It would be good to get #1946 in this release as well.