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