Missing credentials API fix

[jonathanKingston]

Task/Issue URL: https://app.asana.com/0/1199178362774117/1201249917946402/f
Tech Design URL:
CC:

Description:

On imgur.com they're expecting the credential API to be present, probably based on UA string.

This is pending agreement on the config key and a PR to that repo but other than there is no blocker to merging.

There's more copy and paste here for the config checking, we could make part of this PR a separate file that we read in an external file, something like:

        return CredentialsUserScript.loadJS("credentials", from: .main, withReplacements: [
             "import * from 'utils';": CredentialsUserScript.loadJS("utils"),
             "USER_UNPROTECTED_DOMAINS": "",
             "CREDENTIALS_EXCEPTIONS": (unprotectedDomains + contentBlockingExceptions).joined(separator: "\n")
        ])

Steps to test this PR:

1. load imgur.com

Expected
Site to have content

Actual
Site is blank

Testing checklist:

• Test with Release configuration

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Internal references:

Software Engineering Expectations
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[jonathanKingston]

/cc @alistairjcbrown @samsymons (seems I can't add more than one reviewer)

[SlayterDev]

Looks good to me once the config change is in.

In regards to the script imports I'm open to it if you want to do it as part of this PR but I feel that's a decision that should also have @tomasstrba's and probably @samsymons's eyes.

[tomasstrba]

@jonathanKingston, please, do you want to merge this today for F&F release build?

[SlayterDev]

I approved this earlier but was waiting for the Remote Config change to land.