[routes] adapt CookielessConverter to new werkzeug router

The new router does not perform correctly when using regex groups. Moreover,
it is now required to explicitely tell it the converter can match the forward /
using ``part_isolating``.

[docs] update git branch to clone

Merge pull request INGInious#827 from anaHue/master

add - some packages to base Dockerfile

[pymongo] set dependency to v3.x

[flask/sessions] fix the way lti endpoint is checked

Co-authored-by: Anthony Gégo <[email protected]>

[js/studio] fix undefined collapse variable

lti/cookieless : correctly handle sessions (INGInious#685)

* lti/cookieless : correctly handle sessions

This fixes erroneous code where session fetched was still the
cookie one if available and never replaced.

Flask doesn't deal at all with session hotswap and its context
stack, therefore the session manager creates the new session
when it detects the LTI launch page is requested.

* cookieless: do not hardcode lti launch page url

docs: update install doc

docs: update v0.6 branch name

Remove dev from v0.6 version string