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

Remove dev from v0.5 version string

Update badges

Add random input doc

Bump version for release (no bug support on this release)

Update to latest version of Docker; update documentation

Version bump