Allow deleted sketches in collections to be removed (fixes #1465)

[andrewn]

After the discussion about this bug in #1465, I decided that deleted sketches in collections shouldn't disappear. Instead, they should appear in the collection as "Deleted". The user can then decide if they want to remove the item from the collection.

I think this is less confusing and also means we don't have to complicate the sketch-deletion code with finding and deleting collections that reference the deleted project.

I have verified that this pull request:

• has no linting errors (npm run lint)
• is from a uniquely-named feature branch and has been rebased on top of the latest master. (If I was asked to make more changes, I have made sure to rebase onto master then too)
• is descriptively named and links to an issue number, i.e. Fixes #123

[andrewn]

@catarak Since this is a fairly serious bug (the collection page containing a deleted collection doesn't load), I think it's worth merging and leaving #1465 open?

[catarak]

@catarak Since this is a fairly serious bug (the collection page containing a deleted collection doesn't load), I think it's worth merging and leaving #1465 open?

makes sense to me!