editor.js works in a new way with qualitychecks data
Fix missed replacement
Add untranslated qualitycheck to excluded
Cleanup: PEP8 and coding style fixes
Restored `ajax_required` decorators
Simplified code
Added comment for later review
Cleanup: fixed import order
Moved `TreeItem` to `pootle.core.mixins`.
Use a new-style class
Fixed wrong reference
Use a namespace class for referencing cached method names
Braces should be used around all statements, even single statements
Cleanup: removed dead code
Added ajax support to `get_path_obj`

This somehow covers part of what `get_xhr_resource_context` did, and
supports arbitrary paths too.
Pass `dir_path` and `filename` as kwargs
Arbitrary path support for checks' and overview stats
Fixed `get_checks()` call
Provide qualitycheck category names to all editor instances
Minor cleanup