feat: setting a list of user login that can see Administration

What does it do?

This feature permit to set a list of user login that will be able to see the Administration link. If the list is empty every users will be able to see the link (like before without this feature). Maybe a start with CAS auth (or LDAP ?) to filter access to some parts without to request the etherpad settings ?

Is there a related issue?

What part of MyPads are modified?

• backend
• frontend

Check list

• did you squash your commits?
• did you respect the coding guidelines?
• did you add new translations key?
• did you add new tests