add model constraints (mostly validators) and enforce them

This PR adds many constraints to the models to improve the integrity and security (because some user-provided fields may contain dangerous values, eg: Webapp.docker_name, Job.param, ...)

Fixes #223 (closed)

The added constraint mainly consists of adding validators or removing blank=True or null=True when it is not meaningless to have an empty value.

There a few other changes:

• JobQueue.is_default and WebappVersion.published were transformed into BooleanField (because they actually are boolean)
• the sshkey validator was moved from the UserForm to the User model and rewritten to support the actual format supported by the ssh container
• I relaxed the constraint on WebappVersion.url to make it optional (because i do not see a reason why we should mandate users to provide a url for every webapp version)
• enforce model validation when creating a job via the API and when creating a Webapp