Add metadata to identities using flag ids as key

[leo848]

The best option would probably be to separate flags from their related identities and just assign them a unique id that could also be used in the metadata object.
It should include (bare minimum):

• A title (type string)

• Aliases (type string[])

• The 'main' flag (type Flag)

• Alternative designs (type Flag[])

• Further information (?:)

• Give all flags a unique id (bi -> bisexual to not conflict with bigender)

• Add an example metadata entry