Make preview modal tooltips translatable by translators

[chrisgarrity]

Unfortunately the babel tool that automatically extracts localization messages only recognizes defineMessages(...) and <FormattedMessage ... />

Uses of intl.formatMessage(...) won't throw an error, but the string definition will not get extracted by the tool and therefore will not be available to be translated. For example:
https://github.com/LLK/scratch-gui/blob/2753c00a14e998fe30e2b0f2caa2568f64c8fc96/src/components/preview-modal/preview-modal.jsx#L55-L59

Instead we should move the message definition into a defineMessages call, and then call intl.formatMessage with the data from the messages object.

[apple502j]

I do!

[thisandagain]

Resolved in #3257