Partially migrated Tenancy to the new design

Took the old design and moved it to the new design, while trying to
preserve the old desired behavior.
Added #demoted_tenancy? from DemotedTenancy
Added demoted attributes to tenancy

As well as adding attributes I've added validation constraints
Added a new attribute to Tenancy and a new method

Added: - assured_shorthold_tenancy_type
       - #assured_tenancy?
Validation on Tenancy's start_date
Added Tenancy#one_tenancy_agreement?
Only validate start_date if there are multiple tenancies
Added conditional validations

added validations when the user has one & many tenancies
Fixed the examples for assured_shorthold_tenancy_type
Added constraints for reissued_for_same_property

within Tenancy model
reissued_for_same_landlord_and_tenant validation

added to Tenancy
Added the base Tenancy#as_json structure

This is a "target" we should aim for. Gives us common ground to work
towards.
removing duplicated spec setup + refactoring
Added 'demoted_tenancy' to JSON output
First draft UI changes
removed old files
further removal of duplicated spec setup
CSV parser rewrite and new data
updated local pdf results for data scenarios
started on Tenancy#as_json