Update bedrock's cookie helper lib

[alexgibson]

Description

Bedrock's cookie helper is a small custom library, copied from MDN, which was originally based on the source code of this github repo: https://github.com/madmurphy/cookies.js

The version in bedrock, whilst altered for some custom functionality, is now quite out of date. It does not support setting the sameSite property, for example: https://web.dev/samesite-cookies-explained/

We should consider updating our helper, so that we can be more explicit when it comes to setting cookie values relating to 1st / 3rd party contexts. We could either update our existing helper to add support, or alternatively deprecate it in favor of using the cookies.js open source library directly.

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💛 Success Criteria 💛

• Cookie helper is updated.
• Cookies set using JS explicitly set the sameSite attribute (lax should be fine for front end needs where cookies are used to control display of content?).

[mitchell-frost]

I am new to mozilla. Can I work on this?

[alexgibson]

Changes to SameSite cookie behavior are rolling out in Chrome and soon also in Firefox. https://hacks.mozilla.org/2020/08/changes-to-samesite-cookie-behavior/

From what I can tell, I don't think any cookies in use on mozorg are effected directly by this change, but it may still be worthwhile updating this lib, so that we can set the SameSite attribute explicitly, instead of relay on the new implied default.