Only dev *or* prod config, not both

[g105b]

Sometimes it's useful for the code to know whether we're in dev or prod mode. Currently, there is no way to know which mode we're in.

On the development environment, there can be an optional config.dev.ini that will override the default values in the config.ini. The expectation is that the dev ini does not get checked into version control, as this is where the actual keys and secret values will lie.

However, on dev it might be that you use "mock" versions of API keys, or have specific low-exposure keys generated for each developer. When the code gets to production, it would (and should) always be an error if the dev ini was found on the production server.

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Proposed changes:

1. throw an exception if there are both dev and prod at the same time.
2. assume "prod" by default (for code reasons), so things like $config->getDeployment() will return the appropriate enum, and only assume "dev" if there is a config.dev.ini. This will mean that assuming prod by default can apply more strict security / not bundle source maps / etc.