A field widget that allows for entering of sensitive information that can be revealed at the user's request - ie. API keys, secrets.
When a sensitive field that has been previously populated is loaded again, a placeholder is used instead of the real value, until the user opts to reveal the value. The real value is loaded via AJAX.
Credit to @tomaszstrojny for the original implementation.
Replaces #5062. Fixes#5061, #1850, perhaps #1061.
Co-authored-by: Tomasz Strojny <tomasz@init.biz>
Co-authored-by: Luke Towers <github@luketowers.ca>
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