Unify configuration

[fnando]

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Right now, global vs local config is confusing. This is what @leighmcculloch proposed, which seems like a good compromise:

For P23, I think we can:

• Default to the one global location.
• Keep --config-dir which would allow someone changing that one location if they really needed.
• Add a warning to display on use of --global saying it’s unnecessary, global is always on, the flag does nothing, and the flag will be removed in v24.
• Don’t allow a way to choose local.
• Read from both local and global still, but everytime a read reads from a local file, output a warning saying it’s going away in P24.

For P24:

• Remove --global flag.
• Still read from local, but hard error when reading and finding data in local that would be used.

For P25:

• Stop reading from local.