Rename add-on -> apps folder in apps configuration (config.yaml) and mount location

[agners]

There's a couple more. We also have an addons folder and an addons_config folder. This also made me realize a couple other challenges we have to think about:

1. The word addon appears as a mapping option in addon configuration. We'll have to deprecate that
2. Does renaming the addon_config folder may break existing containers with it mounted or will they be fine? Something to test
3. By default all_addon_configs is mounted at addon_config and local addons are mounted at addons in other addons. This will be a tough one to change

Originally posted by @mdegat01 in #6745

Probably the easiest is to introduce new mapping options, and with them mount to the new location. This makes for a clear migration path for existing apps.