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
Update link to bug tracker in README.rst

http://bugs.locamotion.org directs to the GitHub issues page for Pootle.
Bugs: remove some bugs.locamotion remnants
Docs: use #pootle-dev for dev IRC channel
Modify docs for "Committing".
Migration cleanup: Specify dependencies just once
Migrations: Rename migrations to just have one migration per number
Migration: Actually add the field and rename the migration

The other code, kept from the original commit, is no longer necessary
since we removed that field when we switched the User model.

The migration is renamed since it just adds a new field.
Get rid of remaining scripts.actions related scripts
Cleanup: Fix indentation so pep8 passes
versioncontrol: Move to_podir_path outside of versioncontrol
translationproject: Check for disabled projects in more places
Move pootle_misc.versioncontrol to versioncontrol.utils
versioncontrol: Merge pootle_app.vcs with versioncontrol.utils
versioncontrol: Recover some lost code from e23642d8
Minor css fix sidebar

Reported by @julen in f623fb1