Node might be of a text type and thus not have the `getAttribute` method

This fixes the JS error that occurred on permissions page
User: Inherit PermissionsMixin for our custom User model

AbstractBaseUser, which we inherit from, does not provide "has_perm". This is
taken care of by PermissionsMixin which only AbstractUser has by default.

Incidentally this gives us back is_superuser so we can get rid of it from our
class. No migration needed.
Keeping in mind it *also* brings us groups, which is not so great since we
don't actually use those anywhere.

Fixes #3286
Add store field to Submission model

Cherry-Picked-From: 79c7e06f50c3393b6750901f13032789ee9d36c7

Pick notes: Had to update last_submission not to clash with the new fields.
Django 1.6: Replace get_query_set() by get_queryset()
Don't delete Store and Directory objects anymore

They'll become obsolete instead, thus allowing to keep units around, which
are used for keeping track of translator's activities or for generating
invoices.

Fixes evernote/pootle#269.

Cherry-Picked-From: b47cd81ac6ae06142de355f5bd0a529a88cd6a92

Conflicts: *.py

Fixes #3295
Finish getting rid of djblets

Note that this breaks upgrades from version of pootle older than 2.6
Cleanup: removed deprecated dependency checks

Pick notes: This completes the cherrypick started in commit
56605665b3599a872d730aaa8d9e549c2810392f

Cherry-Picked-From: 4dda9c3183f717415fed524d8cecf8984b5799c3
Move all depcheck logic to pootle.core.diagnostics
diagnostics: Move core dependency checks to pootle_app.__dependencies__

This allows a more decentralized approach to dependency checking where each
module is responsible for its dependency checks.
The non-core dependencies still need to be ported in a later commit.

Note that this has a mild performance impact and this logic should only run
once per instance startup.
diagnostics: Check for chardet dependency

Fixes #1719