New Memberlite admin menu area and tools to import, export, and reset

[kimcoleman]

All Submissions:

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

Changes proposed in this Pull Request:

This update moves the former "Memberlite Guide" to a separate admin menu section directly under the Appearance menu.

Menu sections include:

• Dashboard - this is primarily what was on the Appearance > Memberlite guide before.
• Custom Sidebars - this is what was one the Appearance > Custom Sidebars page before.
• Tools - this is newly added in this PR.

The PR restructures how the admin pages are loaded separate of their actual functionality to help better organize the code and make things more clear to work with and add to going forward.

The Tools screen now supports:

• Export Theme Settings: This feature exports the theme customizations, custom sidebars, sidebar CPT assignments, custom CSS, site logo, site icon, etc.

• Import Theme Settings: This feature imports a properly formatted Memberlite theme export file, overwriting the theme options, icon, logo, sidebars, and CSS.

• Reset: This clears out all things that the import/export also includes. It requires a JavaScript confirmation before completing. That process is irreversible.

Other information:

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

Changelog entry

FEATURE: Export and import tools for Memberlite theme settings (Customizer options, custom sidebars, CPT sidebar assignments, custom CSS, site logo, and site icon). FEATURE: Theme Reset tool that restores all Memberlite settings to defaults, with confirmation prompt. ENHANCEMENT: Reorganized Memberlite admin area to main admin menu section 'Memberlite' with subpages.

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