Used MySQL fulltext search to improve search performance (#1476)

no ref

We use MySQL fulltext search to narrow down the result set based on the search
term, then use our previous `LIKE` based ranking system on the reduced result
set

Author: mike182uk

migrate/migrations/000079_add-fulltext-index-for-account-search.down.sql

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+DROP INDEX idx_accounts_fulltext_search ON accounts;

migrate/migrations/000079_add-fulltext-index-for-account-search.up.sql

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+CREATE FULLTEXT INDEX idx_accounts_fulltext_search ON accounts(name, username, domain);