AI expression topics overhaul

[bobular]

This is branched off ai-expression #1330

• accommodate new response with topics field instead of sections.
• remove empty sections (when AI returns nonsense dataset_ids)
• messaging "can take up to three minutes" after button is pressed
• mouse pointer
• gatekeeping based on rnaseq_dataset_count and microarray_dataset_count attributes (can be null or ints)
• try to fix unreliable link-to-expression-graphs-row
  • deal with first-time open issue - fix by opening the transcript expression table when the AI section is opened.
• maybe do the feature flag in a new PR - WON'T DO