Add an option to sort by relative change in the entity selector if applicable

[sophiamersmann]

Core problem

• Some charts have relative mode but we can't currently sort by the relative values in the entity selector, which makes it difficult to discover trends in relative terms
• For example, in this chart, it would be useful to be able to sort the relative number to find countries with the highest relative increase/decrease in military spending

Proposed solution

Add an option in the sort dropdown, something like:

• Military expenditure (absolute change)
• Military expenditure (relative change)

[sophiamersmann]

We're not sure how tricky this is. If it takes more than a ~1 day, then maybe not worth the effort.