Display a limited amount of dependencies and reverse dependencies

[tchoutri]

Package pages must not display more than 6 reverse dependencies, and on a separate line give a link that redirects to a /search query that searches something like dependents:packageName.

This can be generalised for the full view of dependencies, because we love to depend on a bunch of packages, with a dependencies:packageName. See #35

lens will be a very interesting test to see how the UI reacts to a lots of dependencies and dependents.

In the absence of full-index import, base is the canonical example.

[tchoutri]

closed by #81