Initial work on dedup/merge

[JeremySkinner]

Initial work on the deduplication UI. The UI has been ported from the D7 opencrm_dedupe module.

Highlights:

• Implements the merge UI using the D8 form and entity APIs
• Uses events instead of hooks to register fields for display (see GetPropertiesToShowOnDedupScreenEvent)
• Modifies the DashboardNegotiator so that the contacts theme doesn't always have to be applied (this is still the default behaviour, but can now be overridden in the route)

Next steps:

• This needs wiring up to the actual deduplication logic in the entity_merge module (not yet ported)
• This could possibly move to a separate module
• Needs a sensible set of default fields (currently uses mail and gender for testing)

[yanniboi]

Great stuff @JeremySkinner a solid start! :)