layout: Add an alert placeholder

We will use this for notifications that an action has been successfully
(or not) completed.

It will be displayed on the top right of the screen, like desktop
notifications often are.
models: New 'downstream' attribute for packages

This will be used to indicate that a package exists exclusively in the
downstream distro, and as such does not need to be compared with an
upstream one.
ressources: New package resource
views: Allow marking a package as downstream-only
sync: Ignore downstream-only packages

They have been marked as existing only in the downstream distro, so
there's really no point in trying to determine the upstream they came
from, and whether they are up to date.
status: Don't show downstream packages as problems

They aren't problems, we know it: we marked them as existing only in the
downstream distro.
overview: Show downstream-only packages separately

Now that these aren't problems any more, we need to not count them as
problems, and give them their own count.
problems: Allow marking a package as downstream-only
views: Report status of downstream-only packages
views: Allow unmarking a package as downstream-only
downstream: Allow unmarking a package as downstream-only
models: New 'upstream_pkgname' attribute for packages

This will be used to indicate that a package in the current distro has
in fact been renamed from its upstream.
views: Allow marking a package as renamed
sync: Rework the get_upstream_evr function

The code was a bit messy, due to the very long variables and indexing of
a dictionary of dictionaries...
sync: s/pkg.name/pkgname

Next commit will actually depend on that new variable.
sync: Properly handle renamed packages

They have been marked as being renamed from upstream, so when querying
for the upstream EVR, we should do so using the proper name of the
package, as it exists upstream.
status: Display the upstream name of renamed packages
problems: Allow marking a package as renamed
overview: Add a entry for the renamed packages

Now that these aren't problems any more, they are displayed along
regular up-to-date / out-of-date packages.

Nevertheless, it can be interesting to view them all in the same place.
status: Report status of renamed packages