Set "trusted source of title metadata" value for local KB data sources

Following ERM-793, this story is to add the ability to set the "Trusted for title instance metadata" field on local KB data sources and individual jobs

Modify "trusted source for title instance metadata" for a source

• This will be controlled through Settings -> Local KB Admin -> External data sources
• Any user who can edit data sources will be able to edit the "trusted source" field
• The checkbox field should be added to existing edit form for a source, with the label "Trusted for title instance metadata" and positioned to the right of the URI field, as shown in the below mockup
  

• The LOCAL data source is currently not editable - it should be possible to edit (but not delete) the LOCAL data source, but ideally only the 'trusted source for title instance metadata' field will be editable (this could be controlled in the UI or possibly just by the backend rejecting edits to other fields for LOCAL)