[ongeki,api] support ongeki rival

There is a new properties in application.properties :

## Limit rival list size for per user, it will be unlimited if this value <= 0 (CAUTION!).
game.ongeki.rival.rivals-max-count=10

There is first time I code with Java (and its frameworks), maybe some codes are bad for reading, please take a code review.

PR contains migration files for Sqlite/mysql/MariaDB , Sqlite/MariaDB are tested and passed, mysql didn' test.

Currently this function is working with sddt1.35 ,some users are also using without any problem.

Users can add/delete rivals though web page in aqua-viewer.

pic

Adding rival is a one-way operation (means rival user won't add user automatically).