Tobira integration: Improve UX for specifying subpage titles when adding new subpages

[dagraf]

When I create a new subpage in the Tobira tab I can specify a title. But when I insert my Opencast series into this subpage the title of the series replaces automatically this title I specified. I get informed about this in a message ("The title of the selected page will be overridden with the title of the series.") that appears pretty far away from where I can specify the title (see screenshot). This is far from optimal in my opinion since users might easily overlook this message and end up frustrated because their deliberately chosen title gets ignored.

Therefore, I suggest a slightliy altered behavior of the form (see also screencast):

1. When I check the checkbox which indicates that I want to mount my series into this subpage I can no longer specify the title of this subpage.
2. Additionally, the textbox for the title of the subpage not only switches to read only but contains also an explanation like "Title of the mounted series" or contains the actual title of the Series (e.g. "All my best cat videos").