Fix numerous issues in the Horizon package

Fix numerous issues in the Horizon package

The Horizon package comming from Ubuntu has numerous issues, and this
patch attemps to fix some of them. There's some less important things
which could be fixed later on.

  * Adds missing symlinks to horizon to fix broken offline compress, which
    made the package uninstallable (LP: #1470143).
  * Ran wrap-and-sort -t -a to make dependencies readable.
  * Run compile-messages at build time, not in postinst.
  * Activates off-line compress.
  * Fix path of secret-key to be in /var (as /usr should be considered to be a
    read-only filesystem, as per Debian policy).
  * Copy the missing angular folder.
  * debian/openstack-dashboard.postinst
    - Re-wrapped the file which was really unreadable in its current state.
    - Redefines --home as /var/lib/horizon, and not /usr/share/horizon, which
      really is not policy compliant (in openstack-dashboard.postinst).
    - Completely remove the collecstatic which should *never* be done in this
      package: we're not using a virtualenv!
    - Completely remove chown of files in /usr. By policy of the FSHS, this
      should *never* happen.
    - Removed chown of /etc/openstack-dashboard. This is useless and
      dangerous in therms of security.
    - Avoids hacks with the secret-key folder in postinst (since we've set
      that by default to be /var in the default config file).
    - Add off-line compress if triggered.
  * Add trigger if the some javascripts are updated.
  * Fixes /horizon/static Alias directives (thanks to I. Yozhikov).
  * Fixes in _variables.scss for /horizon/ webroot
  * Activating OFFLINE_COMPRESS=True, which speeds up a lot Horizon.
  * Fix the way /var/lib/openstack-dashboard/secret_key is handled
    (previously a copy of /etc/openstack-dashboard, now an alias to it).
  * Fix tests to not try to overwrite the package apache config.

Closes-Bug: #1470143
Change-Id: I7de1968ff9c1ddd03a9965aa66fb9407d3f8d1dd