Fix bug in which we tried to mkdir in the source folder (atop existing dirs) instead of the target folder.
Implement request-time folder views. We can browse again!

* Attach an ElasticSearch pool to the app. I'm not positive this is the best place, but it seems to be instantiated only once under dxr-serve.
* Start using url_for() in several places so we don't have to repeat the knowledge stored in route definitions. It even shortens things!
* Refactor folder.html so it takes a single stream of combined files and folders. Saves some repetition.
* Stop showing mod dates and sizes for folders. They're useless and just noise, respectively.
* Along with the previous info, also index files' names (for sorting) and containing folder paths (for folder view queries).
* Stop indexing file entries for binary files until we have a way to display them.
* Start indexing folders into the 'files' doctype (weirdly enough) so we can build directory listings and find path matches in folder paths, even for empty folders.
* Fix a few things with our Flask app setup to let url_for() work for static files and non-default wwwroots.
* Move folder.png to mimetypes folder so we can treat them pretty much like file icons, simplifying implementation.
* Start indexing file paths so we can sort search results by them.
* Store the tree OrderedDict in the generated config file in alphabetical order. This saves sorting when generating the tree menu for request-time pages (and otherwise, too).