Merge tag '1flow-0.28.0.2' into develop

0.28.0.2 1flow-0.28.0.2
create some new variables to have more things handy for scripts.
Harmonize Folder.add_folder(…) to return also a `created` boolean, like all other Django methods do.
small import reordering.
PEP8.
Avoid a double save() at user creation which leads to a DuplicateKey error, whereas it's just a collision.
Avoid crashing after user user creation if Django user doesn't exist.
A new function to register a {class,bound} method as a celery task, to save a lot of typing and useless glue code in modules sources. Better than nothing and allows to use methods as tasks without the bugs of the current celery implementation.
Use the new `register_task_method()` function and remove glue code.
Add the default_retry_delay_argument to the register method as task arguments.
PEP8.
Register methods as celery tasks in subscriptions.
Register methods as celery tasks in articles.
Fix the new task syntax in articles.
Use `register_task_method` in authors.
Use `register_task_method()` in feeds, and update names with the new `_task` suffix.
Lookup the new `replace_duplicate_everywhere_task` in the mixin method.
Use the new `register_task_method()` in reads.
Add a missing method.
Mini-fix for methods as task in reads.