[setup] temporarily restrict to requests<2.2.29

[frontend/submission_manager] catch DocumentTooLarge exception

Fixes #697.

This is a first mitigation and in all case useful, as the limit is set on the
total command sent to the MongoDB server, which cannot be computed before.

[course_admin] filter non activate and "empty username" users. (#919)

[frontend/tasks] fix call for task_user_can_submit

[backend] BackendJobDone should be sent to the job initiating client

Otherwise in database-shared multi-client (multiprocess) setup, another
client can kill a job it does not own and receive a JobDone it cannot process.

[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 #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 <anthonygego@users.noreply.github.com>