Add accept forms for site-wide licenses on NeuroHub side

[prioux]

A CBRAIN site can be configured to have site-wide licenses or agreements. New users are required to accept the licenses before accessing the rest of the site.

The neurohub side currently bypasses the licenses and allow the users to proceed with the rest of the functionality.

See this for the placeholder code: 0aaa894

Ideally, we want users to accept licenses even on the NeuroHub side. So we need a form that show the licenses and force the user to accept them.