[FEATURE] Added fluidpages sub inheritance

Enables an additional "Page configuration" field with a Flux form which gets used if the "Page layout - subpages" field is set to use another template than the current page. Filling in this additional Flux form inserts template variables which will be used by subpages only.

Additionally, both "Page configuration" fields have been moved to a tab of their own.
Page properties update warning

Please note that a fallback has been implemented which makes fluidpages read variables exactly like before this change right up until you save your pages. This means that when you save pages after installing this feature, you must make sure the "Page Configuration - subpages" field is configured. If you do not ensure this you risk sub pages inheriting empty or unexpected values (since variables now come from the secondary field).
Database update warning

This feature requires running the install tool's DB comparison wizard or manually adding the field documented in ext_tables.sql.