Klaviyo user creation

[njparadis]

First pass at streamlining the initial setup process for M2:

• Added a section to the extension config page to automate the REST user creation process
• Sensitive fields in the config page are now encrypted and obfuscated in the frontend

[njparadis]

Sensitive data updates

Obfuscated private API key:

Obfuscated password:

Error/success messaging examples

User already exists:

Invalid email:

Crappy password:

Success:

[remstone7]

looks awesome, some comments

[remstone7]

assuming the deletion process would still be in the rest role section?

[ihor-sviziev]

@njparadis @remstone7
This is actually not an REST user, this is admin user, that has full permissions to the admin.

It should NOT be inside Stores => Configuration due to security reasons.

Also saving admin user credentials isn't secure, even with encrypting, as this password could be decrypted.