Add base64Encode handlebars helper

[fishface60]

Identify the Bug or Feature request

closes #4924

Description of the Change

This adds a handlebars helper called base64Encode that uses the same encoder as the macro, which happens to use the right parameters to pair with the browser's atob function.

Possible Drawbacks

I have not included a base64Decode function because I could not think of what it would be useful for, it's fallible if given the wrong input and would be hard to handle errors in handlebars so people should do it in javascript.

Documentation Notes

base64Encode

Turns the textual form of the value into a base64-encoded string. For example:

<script type="application/json;base64" id="jsonProperty">
  {{ base64Encode properties[0].value }}
</script>
<script type="application/javascript">
const jsonProperty = JSON.parse(atob(document.getElementById("jsonProperty").innerText));
</script>

Release Notes

• Added a base64Encode handlebars helper to more safely embed data into stat sheets with {{base64Encode value}} and then decode in javascript with atob(document.getElementById(...).innerText).

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