Provide a way to mark relevant packages when in Freeze

[Siegfried Gevatter]

REVU should have a "Feature Freeze" mode which, beside showing a notice about the freeze being in effect and the meaning of that, would enable an additional feature: the possibility, for reviewers (MOTUs) to mark relevant packages (like packages which have a Freeze Exception) so that reviewers know that it's important to look at those packages).

This could be done through an icon like the "favourites" in Brainstorm (ie, shiny if the package has been marked as relevant and of less bright colours when it hasn't, and if a reviewer clicked on it that would toggle the status).