Interactive maps messages review

This pull request was created only for message review purposes. Do not merge it!

Dear ComEng developers: pick red or green background to review the messages and their documentation, please :)

Once you finish review and we address all found issues we'll be able to delete the branch.
If you're [doing it like this](https://github.com/Wikia/app/blob/245a7765/extensions/wikia/WikiaInteractiveMaps/controllers/WikiaInteractiveMapsBaseController.class.php#L147), you should generally pass $1 with `->numParams()` so that it is run through the language specific formatNum. i.e. `wfMessage( 'wikia-interactive-maps-image-uploads-error-file-too-large' )->numParams( $this->getMaxFileSize() )->plain();`.

But, I'd suggest avoiding including your own unit, and instead simply using [Language::formatSize](https://github.com/Wikia/app/blob/dev/languages/Language.php#L4008) (i.e. `$this->wg->Lang->formatSize( $this->wg->MaxUploadSize );`) which will handle both formatting the number and translating the unit, which is how the maximum upload size is displayed on Special:Upload (as "10 MB" in English for example).