First version of Special:PageMigration

* Fetches and displays the translation units created by Translate for the given page
* Imports the old translations for given language on paragraph level and displays them
* Feature can be now conditionally enabled by setting $wgTranslatePageMigration to true
* Allows to edit or delete a translation as well as swapping with the unit below
* Allows to save the translations by creating corresponding pages

Change-Id: Ic62ff3fcf52c1862b5d9f8ec14c60e1a00a340bb
Merge "First version of Special:PageMigration"