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]

PRs

• New API endpoint for listing users torrust-index#788

To-Do Index:

• Database layer logic
• Repository layer logic
• Service layer logic
• New API endpoint

To-Do GUI:

• New admin panel tab
• Logic to get the list of users from the index