Add regex for references field

[amaury1093]

One idea was to use Wikipedia's reference system for our own references, cf https://en.wikipedia.org/wiki/Wikipedia:Citation_templates.

We need to add a regex validation on the references field, this might help.

@whirish You proposed to use a whole new Reference model for this. I totally agree too, and it's much cleaner; I just don't want to create too much additional work. If a new model is doable for v1, then let's go for that, and close this issue.