Improve tag search on Problem

[Eason0729]

The state of art for problem search:

1. support text search (using %text%): sqlx doesn't support fts5 or tsvector
2. support tag search (using COUNT(DISTINCT "problem"."id"))

• automatically add tag on addition
• Cannot auto-compelete tag or list tag
• tag would not delete if all problem deleted

periodic tag deletion was chosen because we use read committed isolation level on SQL transcation during tag addition for performance.

TODO:

• write auto-compelete API
• auto compeletion caching
• periodic tag deletion