Show other tagged resources on the same domain when submitting a resource

[toolness]

It's potentially very easy to duplicate semantically identical content, e.g.

• if someone in England tags a resource at /en-GB/ while another in the US tags a resource at /en-US/
• if someone tags a resource at /about/ while another tags it at /

We could avoid duplication if we show other resources tagged on the same domain when creating a resource. This isn't currently very easy to do but I have mentioned some ideas in mozilla/MakeAPI#205.

This can also be alleviated by normalizing non-ambiguous differences in URLs in #22, and by analyzing metadata on the target page to figure out what the page's "canonical" URL might be, as in #4.