Explicit approval when accepting an invitation to join a group

Overview

When a user is accepting an invitation a group, add an additional popup, when it may have to check group approvals.

When accepting an invitation (pressing the "v" added in #1685):

[ ] display a popup with
- [ ] title: "Accepting a invitation"
- [ ] text: "Do you want to join the group ""
- [ ] if require_personal_info_access_approval != 'none' or if 'require_lock_membership_approval_until' !== NULL (you can get the info in get-group service), add text "Membership of this group requires the following authorization(s): "
- [ ] if require_personal_info_access_approval != 'none': display "The managers of this group will be able to view the personal info (firstname, lastname, email, grade, ...) you entered in your profile" (replace "view" by "edit" depending the value of require_personal_info_access_approval) with on/off switch next to it (default to "off")
- [ ] if require_lock_membership_approval_until is null: display "You will not be able to leave this goup until require_lock_membership_approval_until" with on/off switch next to it (default to "off")
[ ] the popup has "Cancel" and "Join group" buttons, the "approve" button is enabled only if all switches have been manually turned to "on"
[ ] on "join group", join the group with all the approvals (including require_watch_approval if needed) .. like it is already done for the moment I think
[ ] the initial group fetching has to be managed with a loader
[ ] the 'join group' button has to display a loader while waiting for the backend response. The button should be disabled during that time. (the cancel can remain active)
[ ] the cancel should just close the popup without doing anything