MavigationModel: make caching TTLs more consistent

* move isWikiNavMessage method to NavigationModel
Memcache::get - make it clear what does $exp = 0 mean
NavigationModel: properly clear the cache when MW message is edited
Merge pull request #5518 from Wikia/CON-2194-local-nav-cache-purging

CON-2194: purge local navigation cache when MW message is edited