fix: Replace ES6 usage for IE11 compatibility

[jhildenbiddle]

Summary

• Replace template literals
• Replace Object.entries
• Replace Object.assign
• Replace method shorthand (e.g. { foo() {} } vs { foo: function() {} })

What kind of change does this PR introduce? (check at least one)

• Bugfix
• Code style update

If changing the UI of default theme, please provide the before/after screenshot:

Does this PR introduce a breaking change? (check one)

• Yes
• No

You have tested in the following browsers: (Providing a detailed version will be better.)

• Chrome
• Firefox
• Safari
• Edge
• IE

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