* Added support for backup to a remote server using rsync

* updated web interface to get data from user
* added way to list files from server

It’s not using the “username” field of the yaml configuration
file to minimise the amount of patches needed. So the username
is actually sorted within the rsync URL.
Signed-off-by: Bernard `Guyzmo` Pratz <guyzmo+github@m0g.net>

* Added ssh key generation upon installation for root user.
Signed-off-by: Bernard `Guyzmo` Pratz <guyzmo+github@m0g.net>

* Removed stale blank lines, and fixed typo
Signed-off-by: Bernard `Guyzmo` Pratz <guyzmo+github@m0g.net>

* fix backup-location lines, by switching it from id to class

* Various web UI fixes

- fixed user field being shadowed ;
- fixed settings reading comparaison ;
- fixed forgotten min-age field.
Signed-off-by: Bernard `Guyzmo` Pratz <guyzmo+github@m0g.net>

* Added SSH Public Key shown on the web interface UI
Signed-off-by: Bernard `Guyzmo` Pratz <guyzmo+github@m0g.net>

* trailing spaces.
Signed-off-by: Bernard `Guyzmo` Pratz <guyzmo+github@m0g.net>

* fixed the extraneous environment
Signed-off-by: Bernard `Guyzmo` Pratz <guyzmo+github@m0g.net>

* Updated key setup

- made key lower in bits, but stronger (using -a option),
- made ssh-keygen run in background using nohup,
- added independent key file, as id_rsa_miab,
- added ssh-options to all duplicity calls to use the id_rsa_miab keyfile,
- changed path to the public key display
Signed-off-by: Bernard `Guyzmo` Pratz <guyzmo+github@m0g.net>

* added rsync options for ssh identity support
Signed-off-by: Bernard `Guyzmo` Pratz <guyzmo+github@m0g.net>

* removed strict host checking for all backup operations
Signed-off-by: Bernard `Guyzmo` Pratz <guyzmo+github@m0g.net>

* Remove nohup from ssh-keygen so errors aren't hidden. Also only generate a key if none exists yet

* Add trailing slash when checking a remote backup. Also check if we actually can read the remote size

* Factorisation of the repeated rsync/ssh options

cf https://github.com/mail-in-a-box/mailinabox/pull/678#discussion_r81478919

* Updated message SSH key creation

https://github.com/mail-in-a-box/mailinabox/pull/678#discussion_r81478886