change source and target selects depending of the repository

https://github.com/mozfr/transvision/issues/83

I've placed the repository select in first place in the form. When this select changes the value of the other selects (source and target) also changes with the correct options. That's made with a javacript call that load the different option in the select, options that previously has been generated with php.

As side effects:
- I've added a check in the TMX file load in Utils page to avoid a php error that occurs If somebody change the default target or source option and this option is not in the default repository.
- I've change also the way that we look at the repositories, looking directly in the folder without hardcoded the repository list. I've applied this change also in the download page.