OpenAtlas

Provide functionality to add external reference systems to different instances but using the same API.
This can than be used to e.g. connections to multiple different OpenAtlas or APIS instances.

• Add an API string field (not required) to web.reference_system table and the external reference form.
• Possible values will be "hard-coded" in the settings file. This makes sense because additions there will also need software adaptions anyway.
• Rewrite functions to use API functionality depending on set API value.