Bump handlebars from 4.1.2 to 4.5.3 in /src/ui-client (#309)

Bumps [handlebars](https://github.com/wycats/handlebars.js) from 4.1.2 to 4.5.3.
- [Release notes](https://github.com/wycats/handlebars.js/releases)
- [Changelog](https://github.com/wycats/handlebars.js/blob/master/release-notes.md)
- [Commits](handlebars-lang/handlebars.js@v4.1.2...v4.5.3)

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