Stop hard-coding Gaia versions

I kept two separated commits on purpose: one for Bash scripts, the other for PHP.

## Bash
* Read supported Gaia versions from ```app/config/gaia_versions.txt```.
* Fix a wrong variable name in ```updateGaiaRepo``` when checking if the folder exists.
* Move ```$folders``` from setup.sh to iniparser.sh

## PHP
* Read all supported Gaia versions from gaia_versions.txt
* Add a global constant APP_SOURCES in ```constants.php```
* Add and fix existing tests
* Use new function in Gaia view to get the most updated Gaia version for comparison