Improving the appearance of admin activity email; fixing link to update advanced settings

[kimcoleman]

All Submissions:

• Have you followed the 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:

• Fixed link to Advanced Settings in the admin activity email. The use of single and double quotes in the code was stripping out the link.
• Improved color scheme for accessibility and modern PMPro styling.
• Left aligning most elements for accessibility.
• Updating to rather link to our Slack community over X or Facebook to grow that space.