Bring dxr-build up to speed with the changes we made in the organization of the target folder.

In other words, now the thing gets built in the way the server is expecting it to be:

target/
  config.py
  dot-htaccess (probably going away)
  trees/
    HelloWorld/
    index.html

* Factor out some jinja setup repetition in dxr-build.
* Kill the top-level "server" folder. Turn its contents from string.Template templates into jinja templates, like everything else. No more double dollar signs. This kills substitute_in_file().
Let dxr-serve take relative paths.
Make dxr-build tolerate being run over an existing target folder.

No more folder-already-exists errors
[bug 826482] Fix double slashes appearing in URLs.

These aren't tolerated by the test server anymore.

While I was at it, rewrote build_folder so it's a little (but not much) easier to follow. Took advantage of the filesizeformat Jinja filter so our file sizes are a little nicer (decimals, like the previous author wanted) and our code for dealing with them a little shorter.
Adjust makefiles after moving all the files around.
Remove the unused copy of PLY blowing around in the clang plugin.