pithos: Refactor latest_version_list query
pithos: Add new indexes on attributes table
pithos: Add new indexes on nodes table
pithos: Add new indexes on versions table
ui: Break word for images with large names
docs: Remove obsolete Archipelago documentation
docs: Update upgrade guide for 0.16

    - Include Pithos upgrade notes
pithos: Several PEP8 and test fixes
pithos: Backend refactoring

Remove 'pithos.backends.base' module.
Define backend exceptions in a separate module.
pithos: Provide descriptions for backend errors
pithos: Handle backend errors centrally
pithos: Cleanup 'pithos.api.util'

Use functools.partial to reduce code.