Multiple U2F keys
#1148 (closed) permits the registration of 1 U2F key. This issue propose to register more than one key (inspired by GitLab).
- Store more than one key in _u2f* entries (comma separated)
- Add a _u2f* entry to store a name for the key (comma separated in the same order)
- Modify self registration page to choose which key to remove
- Update manager U2F interface to choose which key to delete