Search pagination: Add support for non-file (text) result types

Created by: slimsag

After https://github.com/sourcegraph/sourcegraph/pull/4796 is merged, the pagination API has a known limitation of only supporting non-file (text) result types. This limitation should be easy to lift, there is just one last decision to be made (how to guarantee ordering when indexed and unindexed results are intermingled) -- I have some ideas here just need to think about them a bit more.