Convert external_http pages setting to bool for rails

What does this MR do?

external_http setting in rails is only used as a boolean value: https://gitlab.com/search?utf8=%E2%9C%93&search=external_http&group_id=9970&project_id=278964&scope=&search_code=true&snippets=false&repository_ref=master&nav_source=navbar

In !4919 (comment 485431592) we set it by default to [] which would be treated by rails as true. This will create a bug when custom domains will be available even though they shouldn't. E.g. see discussion in #5930 (comment 484618255)

We already converted external_https to be just a boolean flag a while ago, I think we should do the same for external_http

I setting ~"Pick into 13.7" because #5930 (closed) was introduced in %13.7 and the previous fix is picked there and ~"Pick into 13.8"(we close to cut-off).

Related issues

Closes #5930 (closed)

Checklist

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