[Task]: Enable fake FXA in local production images

[KevinMind]

Description

Previously the use_fake_fxa() function returned true only if settings.DEV_MODE and settings.USE_FAKE_FXA were both true.
This was meant to ensure we do NOT use fake fxa in production and to make it the default in development.

This was clunky though. There are really not many use cases for setting up fxa locally but even when running production images locally, we were then defaulting to an fxa redirect.. That's annoying.

We can set it up smarter to rely on the presence of a valid fxa client_id.. the default is .. If the value is . then we use the fake client, otherwise the real one.

This enables virtually the same behavior with a lot better defaults.

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