[Feature Request]: Design UI of the merge_ui

[gogonzo]

Feature description

Linked with: #17

1. Provide wireframes and description of how relationship between datasets should be displayed in the merge_ui.
   Inspiration:

   • dm package https://krlmlr.github.io/dm/
   • random image showing foreign keys in a database

2. If merge_srv fails, where errors should be displayed

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