packaging: Debian packaging update

Backport various fixes from Alexandre's work on the official Debian
package:

Debian packaging:
* Add missing libqt5sql5-sqlite dependency
* Various copyright updates
* Update Standards-Version
* Priority optional (extra is deprecated now)
* Update Vcs-* fields
* Add missing copyright entry
* Remove useless gbp.conf file (only meaningful when using
  git-buildpackage in a git-buildpackage compatible repo which is
  not the case here)

Packaging scripts:
* Remove Ubuntu 14.04 and 15.10 related code
* Various copyright updates
* Set e-mail address before generating changelog entry

Change-Id: I18e523422d16fe129cf6773ab01a99b00232a5bb
Gitlab: #481
Reviewed-by: Sebastien Blin <sebastien.blin@savoirfairelinux.com>