Fixes #16850: Add config folder for customization files

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 Webservice was separate from MantisBT.  Now it is just confusing.

This change is not ready for merging, but before I do the testing and polishing work, I wanted to make sure we are all on the same page.

TODO for this pull request:
- Testing
- Check.php to flag legacy files.
- Any other locations to flag moving of files?

Follow up work:
- We've discussed the concept of "feature customization" files vs. "environment settings".  Once we have clarity about how this works, we can include it here or in a separate pull request.  We could also consider leveraging system environment variables to control overrides that are dev/test/prod specific.
- I expect we will use this folder to allow users just dropping logo or font files which we would detect and make available.