Implement request-time rendering of files. Fixes bug 1045183.

[erikrose]

• Stop rendering file HTML at index time, and instead insert refs, regions, annotations, and links into ES for use at request time.
• Bump format version. DXR instances now consist of just config.py in an otherwise-empty folder. And most of that will go away once we implement independent tree indexing.
• The generated config.py and the HTTP controllers had a misguided idea that there is one set of enabled plugins across all trees. Corrected that. That would have been a big surprise once we started indexing more trees, with different sets of plugins. The only reason it even ran was that the generating code referred to a "tree" variable, the iterator of a list comp which leaked out (since this isn't Python 3).
• Correct the documentation of refs() and regions(). Neither is actually per line.
• Since HTML is never built at index time, remove the "html" option for "skip_stages".