Refactor tags support out of sideboard.lib.sa

[EliAndrewC]

Moved here from the (now private) original Sideboard issue tracker.

One of our early apps had some special casing for a "tags" attribute in its models. There's one specific place in the code where we check for tags and handle it appropriately.

We don't want to hard-code this into core Sideboard; at the same time, we want that code to work as a Sideboard plugin. So for now I think we can just cut this out of our code, but we should spend just a few minutes thinking about how to make this code extensible, perhaps adding support for the concept of "foreign key into any table" which is really what tags are all about.