Streamline archiving-related methods to hide underlying logic related to deletion

The following discussion from !188510 (merged) should be addressed:

• @rymai started a discussion:

  observation: We might want to override #archived? to super && !marked_for_deletion? to avoid all these additional if/unless project.marked_for_deletion? conditions everywhere. It would also hide the logic inside of #archived? instead of forcing developers to know that when a project is marked_for_deletion?, it's also archived.

  We should probably also introduce an #archiving_available? helper method as !project.marked_for_deletion? && can?(current_user, :archive_project, project).

This is how the #archiving_available? method would be used: !188510 (comment 2456030233)