Add extension support for non-english names

[damianhxy]

Description

Replace use of btoa with custom base 64 encoder obtained from MDN.

(Note that it uses the deprecated substr method, I've replaced it with substring which is equivalent since we start from index 0).

Motivation and Context

Currently, it is generally not possible to create an extension for students with non-english names.

For reasons (see #931), we are base64 encoding student names and looking them up in a dictionary. However, btoa "expects to be passed binary data, and will throw an exception if the given string contains any characters whose UTF-16 representation occupies more than one byte."

On the other hand, Ruby's Base64.strict_encode64 handles it just fine. Hence this PR replaces the base64 encoding algorithm on the frontend.

Closes #1466

How Has This Been Tested?

• Create a student with a chinese name (e.g. 小明)
• Give student an extension successfully
• Also test that we can still give extensions to students with english names

Types of changes

• Bug fix (non-breaking change which fixes an issue)

Checklist:

• I have run rubocop for style check. If you haven't, run overcommit --install && overcommit --sign to use pre-commit hook for linting

[victorhuangwq]

Works as expected.
LGTM

[victorhuangwq]

works with 😀 now as well