Add ability to download schema as an xml

[maxpatiiuk]

Database schema is stored as an XML
XML is parsed by the back-end and sent as JSON to the front-end.
It would be useful to also have an endpoint for getting back-end schema as an XML.
That would be useful because then you would be able to add a "Download as XML" button to the Schema Viewer page.

TODO:

• Add an endpoint for accessing data model as an XML
• Add a button to download the XML data model to the Schema Vieewr

[maxpatiiuk]

Turns out an endpoint for this already exists: /static/config/specify_datamodel.xml
In fact, you can access any sp6 files at /static/config/ url

[specifysoftware]

This issue has been mentioned on Specify Community Forum. There might be relevant details there:

https://discourse.specifysoftware.org/t/specify-7-8-7-release-announcement/1079/1