Switchers handled by docsbuild-scripts.

[JulienPalard]

closes #90

Tasks:

• Disable switchers if not disabled cpython side.
• Install own version of switchers.js
• Make switchers.js find the switcher place, even in the absence of span placeholders (cpython could help by adding proper ids, but it's also nice if it works on old branches in which we won't commit the proper ids).
• Install own version of indexsidebar.html
• Template switchers.js so var all_versions and var all_languages are not duplicated between build_docs.py and switchers.js.
• Template indexsidebar.html.
• Find a better way to insert our switchers.js in the generated HTML pages.

[JulienPalard]

Looks like it works (local builds are usable, tested using ./build_docs.py --quick --build-root build_root --www-root www --log-directory logs --group $(id -g) --skip-cache-invalidation as documented in the README, on my laptop, beware it take hours).

The current implementation works without modifications on cpython, so it'll work on old branch we don't want to pollute with this, and we'll be able to clean the master branch later if this PR gets adopted.

Please let me merge this PR myself, so I can closely watch how it goes on docs.pyhon.org immediatly after the merge (as salt will deploy this automatically after merging), to catch potential issues as early as possible, but reviews are appreciated!