mysql: improve backslash-handling in dashboard variables

if you want to use a dashboard-variable in a query, and the variable-value contains a backlash, it will not be handled correctly. for example: - you have a table (`t1`) with a `name` field, that contains the value `tw\o` (4 characters). - you create a dashboard variable, name=`v1`, type=custom, with two values: `tw\o` and `tw\\o`. - now write a query like this: `select name from t1 where name='$v1'` - if you choose the variable-value `tw\o`, this returns 0 rows - if you choose the variable-value `tw\\o`, this returns 1 row we should look into improving this (so that the value `tw\o` finds the database-row. complications: - we have to be careful, someone may already be working around this problem by duplicating the backslashes, and us escaping the backslash might break such dashboards (but it may be the right thing to do, needs investigation). - the rules change when the variable is interpolated into a regular expression the mysql-database behavior also slightly changes based on the value of the [NO_BACKSLASH_ESCAPES](https://dev.mysql.com/doc/refman/8.0/en/sql-mode.html#sqlmode_no_backslash_escapes) setting, ideally we should find a solution that works always, regardless of this settings's value. originally discussed in https://github.com/grafana/grafana/discussions/38343