Fixes #16850: Add config folder for customization files

Fixes #16850: Add config folder for customization files
Fixes #16446: Merge MantisConnect configs into MantisBT standard configs.

In preparation for our next release, it is a good time to re-organize how to organize customization files.  This change adds a config folder that includes all custom_*_inc.php files and config_inc.php file.

I've also merged the SOAP API configs into our standard configs.  The fact that we had separate configs was from the days where MantisConnect web service was separate from MantisBT.  Now it is just confusing.

This change includes the addition of a configurable config path that can be set via an environment variable or defaulted to MantisBT config subfolder.  The change also includes all the necessary validation in admin/check/ and admin/install.php.