Don't use pkexec for flatpak

[axtloss]

Flatpak is executed with pkexec in installation.js line 110 and installation.js line 119. This really isn't a good idea, it'd be better to execute it without pkexec and using the --user flag. Another thing is that maybe it'd be better to just add the flathub repo as flathub instead of flathub-modren, since having two repos (assuming the user added flathub before) with two different names just makes stuff more complicated.