Add length constraints

[leasunhy]

Currently there are no length constraints for fields in several forms. We should add them because:

• Login names, passwords, etc, should not be too short.
• Submitting a very long data to a field may cause 500 since there are length constraints in corresponding models and the database transaction will fail committing.

Fields involved include but not limited to:

1. User: login_name, password, note, nickname, etc.
2. Post: title.