Add link to mappings

*Please do not deploy this until the code in https://github.com/alphagov/transition/commit/67bac50c31a17d9fb74013743b011a0a1c31f2e7 has been deployed.*

As per https://www.pivotaltracker.com/story/show/64748980, this creates a link on the header of the side-by-side browser for users to be able to edit the mapping they're looking at.