React App Logout and User Menu in Single App Mode

[ckrew]

Description

This merge request addresses an issue when react apps are in single app mode. Currently there is no way for the user to log in, log out, or view the admin pages. This PR adds these elements to the react app header, in the same styling as the log in and user menu in a normal tethys environment

Changes Made to Code

• package.json: updated to add more dependencies for the dev server
• Header.js: added some conditional statements for header buttons. In multiple app mode, put an "Exit App" button. In single app mode, if a user is not signed in, add a "Log In" button. In single app mode, if a user is signed in, add a dropdown to access locations like user profile, admin settings, and log out
• UserHeaderMenu.js: The user dropdown button to access locations like user profile, admin settings, and log out
• react env template: added a TETHYS_SINGLE_APP_MODE env to track when in single app mode
• tethys_portal/views/api.py: Updated how the APIs handle when in an open portal. If the portal is in open portal mode, then don't throw any Unauthorized (401) errors. Also added a gravatar url to the get_whoami api so that the react app can use it in the user header menu.

Quality Checks

• At least one new test has been written for new code
• New code has 100% test coverage
• Code has been formatted with Black
• Code has been linted with flake8
• Docstrings for new methods have been added
• The documentation has been updated appropriately

[coveralls]

coverage: 100.0%. remained the same
when pulling 1bd1526 on reactapp_standalone_logout_and_user_menu
into 582989c on main.