Remove Bootstrap, fonts, and descriptions from admin pages

[joshcanhelp]

Changes

• Remove outdated Bootstrap JS and CSS from admin screens (corrects a number of display issues that appear in v3)
• Use default WordPress admin font
• Removed Enterprise setup option (was showing an outdated list of enterprise connections, did not provide any additional functionality in the plugin)
• Adjusted the setup wizard steps for simplicity
• Removed class WP_Auth0_InitialSetup_EnterpriseConnection
• Removed protected property WP_Auth0_InitialSetup->enterprise_connection_step
• Removed public method WP_Auth0_Admin_Generic->render_description() and protected property WP_Auth0_Admin_Generic->_description

Note for reviewers: Turning off whitespace changes will help. Please note that the admin documentation did not change much from what it was originally.

Settings screen

Setup Wizard step 1 screen

Import/Export Settings screen

Testing

• This change adds unit test coverage
• This change has been tested on PHP 7.1 and WP 5.3.2

Checklist

• All existing and new tests complete without errors
• All code quality tools/guidelines in the Contribution guide have been run/followed
• All relevant assets have been compiled as directed in the Contribution guide, if applicable
• All active GitHub CI checks have passed

[joshcanhelp]

Lots of whitespace and HTML adjustment. Documentation wording only saw minor changes.