A user should only be able to create pages when he or she is editing a channel

The 'Add page' button in the 'Pages' dialog should only be usable when the channel is being edited. When the channel is in 'View' mode, the state of the button depends on the type of a user:

• a webmaster (who can click 'Edit Channel') will see the 'Add page' button grayed-out, with a tooltip saying why it is grayed out

• a non-webmaster (e.g. an author, can does not even see the 'Edit Channel' button) does not see the 'Add page' button at all