Add language level to spoken languages, re #14

TandemPlugin.class.php

classes/TandemManager.class.php

classes/TandemMatching.class.php

controllers/my_tandems.php

controllers/pair.php

controllers/profile.php

migrations/10_add_level.php

models/TandemProfile.class.php

models/TandemUserMotherLanguage.class.php

plugin.manifest

views/_common/_profile.php

views/my_tandems/manage_mother_languages.php