Remove _search dependency on plaintext columns

[timolegros]

Link to Issue

Closes: #9157
Closes: #9171

Description of Changes

• Removes triggers which used plaintext columns to update the _search column
• Renamed _search columns to search because Sequelize doesn't support underscore prefixed column names
• Created getThreadSearchVector and getCommentSearchVector utility functions

Unfortunately, setting the search column cannot be done at the model level (via hooks/triggers) since in the S3 migration the full-text columns will be removed entirely (full body/content would not be available in the hook/trigger).

Test Plan

• Create a thread and comment
• Search for the thread and comment

Deployment Plan

Other Considerations