[GUI] Re-factoring: Databases and users management

[nuxwin]

The current database schema for the managment of the databases and the associated users is bad.

First, all queries must be rewriten to delete heredoc syntax.

Also, we should use many to many relationship according the SQL good practices. For that, we should create a new database table named 'sql_database_sql_user'.

The new table must provide two columns where the first column should contain the Id of a database and the second, the Id of Sql user.

Indeed, an database can have many Sql users and an Sql user can be used by many databases.

[nuxwin]

First part fixed in r2717

[nuxwin]

@Benedikt: Please, create roadmap for 1.0.7 and shift this ticket in it. Thank.