[#4728] - Confusion between groups, roles and permissions

There is confusion between roles and groups in JasperServer.

For me :
- a role is a set of permissions and not a set of users.
- a set of users is a group.
- one or more roles can be assigned to a user or a group of users.

The roles are in fact listed in the "Assign permissions form" :
- Administer (Administrator)
- Read Only (Reader)
- Read + Delete (a strange use case !)
- Read + Write + Delete (Contributor).

A Read+Write (Editor) role should be available too.

So, the actual implementation is not clear for end users.

I suggest to rename the strings concerning roles to their equivalent for groups.

There are some tests in JSP to check the user's role ( ROLE_ADMINISTRATOR, ROLE_ANONYMOUS and ROLE_USER).
It should be more flexible to check on permissions, not on roles.