collection of small changes for new uploader

Contains:

- amendments to the strategy for assessing whether uploads overlap
- revision of the annotation codes and text we display in tooltips when data is annotated

Not sure who to ping on this... @ianjorgensen or @nicolashery if either of you are actually around, otherwise @jh-bate?

Don't merge yet, as the tests for the first item are still pending; just trying to keep things moving along as quickly as possible.