Show the upgrade notification only in specific views

[benmuth]

Summary

This PR adds back the upgrade notification banner that was originally added in #1274 and removed in 4af9bee, but only to specific views instead of the entire admin site, as discussed in #1306.

I wasn't sure for which views we should show the banner, so for now it's only showing on the snapshot view (the page at /admin/core/snapshot/.

The necessary context is provided by overriding the changelist_view method of the SnapshotAdmin model. I think this could work for all views except for the main admin page (/admin/). If we want to pass context to the view for that page, I think we can override the each_context method of ArchiveBoxAdmin instead.

Related issues

#696

Changes these areas

• [ ] Bugfixes
• [x] Feature behavior
• [ ] Command line interface
• [ ] Configuration options
• [ ] Internal architecture
• [ ] Snapshot data layout on disk