Admins should be able to list users

[josecelano]

Parent issue: #657

Add a new tab to the "Admin Settings" panel to list users:

This first version can only contain: username, email.

In the future, it can contain statistics and other actions.

[mario-nt]

Depends on

• New API endpoint for listing users  torrust-index#796
• New API endpoint for listing users torrust-index-api-lib#49
• Types for new API endpoint for listing users torrust-index-types-lib#28

To-Do:

• New placeholder admin panel tab
• Logic to get the list of users from the index
• Admin panel tab Styling
• Add pagination to the list of results