Added /vendors/add functionality

This endpoint will make a user with the specified ID into a vendor and
create their corresponding vendor object in the database.
Merge branch 'data-model' of https://github.com/rpanjwani/beLocal into data-model

Conflicts:
	src/server/be_local_server/models.py
	src/server/be_local_server/serializers.py
	src/server/be_local_server/views.py
	src/server/server/urls.py