Remove the ObjectAliasTarget model

[Alef-Burzmali]

• Remove the ObjectAliasTarget model
• Replace ObjectAlias.targets with ObjectAlias.prefixes, ObjectAlias.ip_addresses and ObjectAlias.ip_ranges
• Remove unused code that supports the alias targets
• Update the migrations to convert existing targets and create a squashed migration to skip that model in new installs
• Update related templates and forms

The main motivations behind this change are:

• To get rid of all the custom code that was required to make the ManyToMany GenericRelationship work without touching the ipam models
• The ObjectAliasTarget was not always correctly recorded in ChangeLog (only an id was), which broke compatibility with the new Branching plugin.
• The API interactions to create an ObjectAlias was clunky as you needed to manage the ObjectAliasTargets
• The desired feature to be able to add other models in the list of possible targets via configuration turned out to be impossible to implement.