Fix migrations for MP_Tree

The migration mistakenly adds MP_Tree path field as unique before creating the field.
Added migration to make Page.path unique after the data migration
Merge pull request #3537 from jsma/migrations-fix2

Fix migrations for MP_Tree
Fixed "Basic Infos" => "Basic Settings"

This is more consistent with "Advanced Settings". The plural form "Infos" does not read well for native English speakers (it would never be pluralized as "Basic Informations").