New bindable AppContainerModel.intializingLoadMaskMessage

[jacob-xhio]

…sages for hoist apps

Hoist P/R Checklist

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• Caught up with develop branch as of last change.
• Added CHANGELOG entry, or determined not required.
• Reviewed for breaking changes, added breaking-change label + CHANGELOG if so.
• Updated doc comments / prop-types, or determined not required.
• Reviewed and tested on Mobile, or determined not required.
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[jacob-xhio]

[amcclain]

Great thanks - small comments below, also pls add the same to mobile