Make linked datasets read-only

[olsen232]

Avoids confusion of writing non-linked local changes to a linked dataset, which will cause it to be de-linked on a tile-by-tile basis.

Datasets can still be modified by reimporting over the top.

https://github.com/koordinates/kart/issues/905

Checklist:

• [x] Have you reviewed your own change?
• [x] Have you included test(s)?
• [x] Have you updated the changelog?