Some cleanup and modernization.

* Added Finnish (fi) i18n.
* Moved the extension description into a translatable message
  and added a link to the special page to the message.
* Replaced dirname( __FILE__ ) with __DIR__.
* Removed (incorrect) $wgSpecialPageGroups definition from
  setup file in favor of defining the getGroup() function in
  SpecialMultiUpload.php.
* Removed a duplicate $wgExtensionMessagesFiles['MultiUploadAlias']
  definition and added MultipleUpload (old special page name)
  as a valid alias to Special:MultiUpload in order to not break
  link when wikis upgrade to the current version.
* Various documentation and coding style tweaks all over the
  place.
* Removed getVersion() from the API module, getVersion() is
  deprecated and no longer necessary.
* Removed seemingly unused and broken numberOfRows() from the
  SpecialMultiUpload class; the function was returning a global
  without declaring it as such, hence why it was broken.
* Removed some superfluous semicolons.

Change-Id: If58206451061985a664473465e66067caa39f3a6
Merge "Some cleanup and modernization."