Fixes User Linking & Creation Issues

[timnolte]

All Submissions:

• Have you followed the plugin Contributing guideline?
• Does your code follow the WordPress' coding standards?
• Have you checked to ensure there aren't other open Pull Requests for the same update/change?

Changes proposed in this Pull Request:

• Closes How to disable the auto creation of users? #157
• Closes . and - are cut from the preferred_username #187
• Closes Authenticate by email if $subject_identity doesn't match #212
• Closes username-transliteration-failed on alpine docker wordpress #214
• Closes Attribute Casing issues #247
• Closes Error Identity not map existing user: User Identity not linked to an existing Wordpress user #249
• Closed Error: User identity is not linked to an existing WordPress user. #278
• Fixes Settings Page typos & formatting.

How to test the changes in this Pull Request:

1. Setup the OIDC Client settings with any combination of "Link Existing Users", "Create user if does not exist", "Identify with User Name".
2. Login with a configured IDP with credentials based on the configured options.
3. Observe the correct client authentication, linking, creation handling.
4. Regardless of the casing of either the WordPress user or the IDP user accounts should properly link when specified.

Other information:

• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you successfully run tests with your changes locally?

Changelog entry

Enter a summary of all changes on this Pull Request. This will appear in the changelog if accepted.

Fixes issues with improperly handling combinations of the "Link Existing Users", "Create user if does not exist", and "Identify with User Name" configuration options. Also addresses issues related to case sensitivity of usernames when attempting to "Link Existing Users".

[zlmitchell]

This change worked for our environment. Thanks Tim.