Parse #<topic> expressions in SearchForm's parsed query text, and support it through the data serialization.

[isoos]

• part of Tracking completion widget issues #8189, follow-up to Rewrite/redirect '#<topic>' search expressions to 'topic:<topic>' #8227
• this does not reverse the topic expression canonicalization of Rewrite/redirect '#<topic>' search expressions to 'topic:<topic>' #8227, but enables that option to be done later
• separated the new tests into two commits, they can be compared before-after the current change

[isoos]

I'm closing this without merging, as it seems to be affecting too many internals, and they are better to be kept clean. I'd focus on doing this transformation only in the user-facing SearchForm in a separate PR.