Allow Multiple Tagging

Hi,

I have added functionality to allow user to assign multiple tags to an answer.

Please merge this pull request.

**Changes**
1. Changed value of `data_version` property in `user:Setting`
2. Changed buckets storage from csv to array
3. Changed type of property `bucket` in `link` object from csv to array
4. Changed `data_migrate.js` to migrate data from v2 to v2.2