Added attributed string for text view placeholder

[blaswan]

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• I've read and agree to the Code of Conduct.
• I've been mindful about doing atomic commits, adding documentation to my changes, not refactoring too much.
• I've a descriptive title and added any useful information for the reviewer. Where appropriate, I've attached a screenshot and/or screencast (gif preferrably).
• I've written tests to cover the new code and functionality included in this PR.
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PR Summary

e.g. New functionality for producing whatsits.

Related Issues

e.g. Fixes slackhq#206 and closes slackhq#230

Test strategy

e.g. Add tests around whatsit production.