For the sake of god, please fix the "casbin_rule" table name bug

[hsluoyz]

We already see tons of complaints from the users for such a boring problem, see:

• Error 1146: Table 'shapeapi.casbin_rule' doesn't exist #82
• The default automatically created table is “casbin_rules“, but the default selected table is “casbin_rule“ #80
• Behaviour of NewAdapterByDB() has a breaking change #78

I'm so tired of it. Such bugs are harming our software quality.

What I want to achieve is:

1. Not a single bug report about the "casbin_rule" or "casbin_rules" table name any more.
2. For a new user, we always use the new table name "casbin_rule".
3. We take care of the DB migration. If the user is using old DB table name "casbin_rules", we will create the new table "casbin_rule" and migrate the data from old table to new table. If the user is using a custom DB table name like "abc", we do nothing.
4. If there's any bugs for newer version of Gorm, then don't update Gorm' version. Don't use a Gorm version with bugs.
5. Fix any existing bugs with a PR. Then make a release.