Support for rsync+ssh backup target (#678)

* 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

setup/system.sh

@@ -214,6 +214,12 @@ pollinate  -q -r
 
 # Between these two, we really ought to be all set.
 
+# We need an ssh key to store backups via rsync, if it doesn't exist create one
+if [ ! -f /root/.ssh/id_rsa_miab ]; then
+	echo 'Creating SSH key for backup…'
+	ssh-keygen -t rsa -b 2048 -a 100 -f /root/.ssh/id_rsa_miab -N '' -q
+fi
+
 # ### Package maintenance
 #
 # Allow apt to install system updates automatically every day.