Empty controller for json ingest
Minor change to execution contexts: use background more places
edit reference styling
tooltips to spreadsheet plus signs
Rename references to links

This hadn't gone into production, so I just did so globally.
offsets can be negative
Provide SQLType[IndexedSeq[_]].put implementation

Was previously falling through to default, which was sometimes wrong,
but 7b5e56987837eae5874e63ed0a7f490e7ad0fa72 (correctly) shadowed this,
exposing this problem.

Fixes #2845, #2849