RFC: line in the sand disallowing potentially unusable components

[sndrs]

What is the purpose of this change?

Proposes a line in the sand that requires Source components to not display if they are not usable.

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Latest commit: 0724cd0674923980c90b2e45bcc15053264e2470

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[sndrs]

@SiAdcock @guardian/client-side-infra i've updated the text to make it expllictly about source (i hope) but it's made me wonder how much this should a line in the sand only for source?

shouldn't we avoid displaying inoperative UI everywhere? i.e. is a source RFC the right place for this?