Widget HTML cleanup

[eelcovdw]

Description

Preparation for ObjectDiffBatch repr + various visual fixes

For new reprs/widgets we need to reuse parts of existing HTML reprs, so we need to split them out/modularize them.

• Move more complex html components to new components module, so we can isolate+re-use them
• Add copy button to table
• fix table alignment/overflow
• rewrite widget header to HTML, re-use diffbatch table HTML

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