Clean up code base, remove unused code and tests

[tmfrnz]

Drop SDG target tables (not needed in this multiframework version):

• tables: sdgtarget_categories, sdgtarget_indicators, sdgtarget_measures, sdgtarget_recommendations, sdgtargets
• controllers
• models
• policies
• serialisers
• specs: controllers, factories, models
• config/routes

Remove any unused Views:

• users
• devise?
• any other?

Remove unused specs

• if any?

[tmfrnz]

• remove unused gems (batch_api ? )
• update gems (if advisable)