New _customizations subsystem

TODO:

• Theme support
• Template support
• Documentation (https://gitlab.com/tikiwiki/tiki/-/tree/master/_custom_dist)
• Deal with current lang/??/custom.php (ouf, that's pretty big)
• Enumerate custom Wikiplugins (@jonnybradley request)
• Move db/config/custom.xml

WONTDO:

Custom smarty functions. Definitely doable, but not urgent enough.