Redmine

This can be very useful to be abble to restrict editability of some projects custom fields.

This patch is based on the visibility roles of issue custom fields.

• This feature is un-useful without another enhancement : givable roles limitation.
  A new boolean field can be added to roles ; givable.
  If the role has a givable value of false, only an admin can assign this role on a project to a user.

This fixes the issue that the project leader can assign himself on a project with any roles, and by the way give himself the role to be able to edit the custom field.

Another use of this second feature is to disable new uses of old roles (for example : related to previous workflows that are deprecated but still used on old projects).

We will provide a patch for this second feature too