Add visual cues to FK column header

[seancolsen]

Fixes #950

Before this PR

Design I followed

I'm roughly following the design in the prototype:

After this PR

Notes

• Small improvisation: I added a link icon to the FK column and a key icon to the PK column to designate the constraints. We can get rid of these if needed.

• I had to make the column header a bit taller to account fot the extra line. I chose to keep its current height when the table has no foreign keys. Not sure if that's the best approach though.

• I'm not sure of the best way to delimit the table name from the column name. In Show detailed FK info in constraints dialog #1383 I used an icon. That didn't look so good here. And I wanted to share the code between the two UI locations, so I converted them both to dots. In theory a user could have a table name containing a dot which could create confusion when reading the table name and column name. Though perhaps that's unlikely enough to discount. In the end, I chose to follow the prototype. I figure we can think harder about this after getting user feedback.

Checklist

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• My code follows the established code style of the repository.
• I added tests for the changes I made (if applicable).
• I added or updated documentation (if applicable).
• I tried running the project locally and verified that there are no visible errors.

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[seancolsen]

Whoops -- This is ready, but should be marked as a draft because it's based on #1383

[seancolsen]

I'm closing this PR per a discucsion where we decided to pursue a simpler design.