Email Tags API.

Copied from the Easy Digital Downloads plugin.
Refactor with NUA prefix.
Initialize the email tags class.
Use attributes for email tag callbacks.
Modify admin notification email.
Use email tags for the approval email.
Remove unused variable.
Move the password reset logic to its own function.
Add the loginurl email tag.
Add a label when returning the username.
Remove unused email tags.
Add email tags to deny user message.
Merge branch 'feature/email-tags' into develop