fix(gatsby-plugin-google-tagmanager): handle line breaks

[stmcallister]

Description

When the line breaks are present in the iframe tag, Google Search Console is not able to verify ownership of the site. This pull request removes the line breaks so that the iframe tag is all on a single line.

Related Issues

This is response to issue #11014.

[m-allanson]

Thanks @stmcallister, this LGTM 👍

If you have the time to do a follow up PR it'd be great to see a test that ensures the single line format for the iframe element. Otherwise we may unintentionally break this fix in a future update.

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[DSchau]

Thanks @m-allanson for the review :)

Successfully published:
 - gatsby-plugin-google-tagmanager@2.0.8
 - gatsby@2.0.96