cli: add simple token management commands

[slint]

Example:

# Create a token
$ invenio oauth2server create_personal_token "my-token" \
    --email foobar@example.com --internal \
    --scopes user:email,records:read
abcdef1234567890abcdef1234567890

# Delete a token
$ invenio oauth2server delete_personal_token --name "my-token" --email foobar@example.com
Token "abcdef1234567890abcdef1234567890" deleted

# ...or...
$ invenio oauth2server delete_personal_token 
Enter token: <password-like input>
Token "abcdef1234567890abcdef1234567890" deleted

(Command/parameter/argument naming could be better, so feel free to change...)

[lnielsen]

Would this be better suited via the admin?

[slint]

Well, why not both? The idea was to have it for API-only apps, which don't necessarily have an admin panel (or access to user settings)