New <dialog> for GitHub Issues

Select all Issues from database where state is open and for this
repository
Update to use $github->config instead of $github->config->database

Also, connect to database before starting switch & after Webhook
validation to avoid having to connect withing each case.
Apply updates to DB connection and enable pagination

- Use $github instead of $github->database
- Write out table in HTML instead of a `\core\table`
- Get start position from `$_REQUEST`
- `LIMIT {$start}, 10`
- Pagination buttons (prev & next)