fix(explorer): show data source defined in explorer configs

[sophiamersmann]

Reverts some changes made in #2794

Bug

• Some explorer charts define source information in the explorer tsv config
• That information is currently not taken into account, and no source is displayed (see screenshots)

What happened

• Source properties used to be spread into the core column definition (as sourceName, sourceLink, etc)
• The explorer's column grammar defines sourceName, sourceLink, etc. and sets these properties on the core column defintion
• In Sources modal: simplify CoreColumnDef #2794, I removed sourceName, sourceLink, etc in favour of defining a source object that holds those properties
• As a result, source information defined in the explorer config are ignored

[danyx23]

Looks good!