Removing word "Medias" from translations

According to http://www.oxforddictionaries.com/definition/american_english/media the word "media" is the plural form of "medium". Therefore, there is no word "medias".
I did not go so far to rename the models and associations, but IMO at least the translations should follow grammar.
Merge pull request #459 from gedsed/patch-1

Removing word "Medias" from translations
add missing label.length translation
Merge pull request #461 from xaben/fixLabelLengthTrans

Add missing label.length translation
Check if file exists when generate thumbnails
Fixed a typo.
Merge pull request #463 from aybbou/master

Fixed a typo.
fix issue with doctrine annotation driver
Updated gallery view layout
Merge pull request #466 from Bladrak/galleryViewLayout

Updated gallery view layout
Fix #400, Fix #402, Fix deletion of File type
Update composer.json
Merge pull request #462 from Prims47/check-file-existence

Check if file exists when generate thumbnails
Fix #464, Fix #180, update the documentation to explain how to use the LiipImageBundle
Fix typo
Fix #393, by adding try catch and log issues
Added missing breadcrumb
Merge pull request #468 from sylvaindeloux/SONATA-218

Added missing breadcrumb
Fixed missing breadcrumb
Merge pull request #471 from sylvaindeloux/master

Fixed missing breadcrumb