Don't allow editing of grant resource or subject

[gsfk]

Simplest possible fix for Redmine 2814: when editing grants, don't allow changes to "subject" or "resource".

• adds an isEditing parameter to some grant form components... this could have been avoided by defining isEditing=!!initialValues but that seems likely to break something in the future
• removed some comments specifying that grants are not editable
• leaves open for the moment what to do about resources that no longer exist, this is arguably a separate issue (the UI already makes it clear that they don't exist, currently it's just up to the user to remove them).