Add own translation key for "Delete Image"

[mantenie]

Currently the button to delete an image is solved by concatenating two translations:

https://github.com/sferik/rails_admin/blob/834c83da05f6194bf808f052e011d539ceaa3845/app/views/rails_admin/main/_form_file_upload.html.haml#L15

The problem is that in other languages, like German, the sequence of the words is grammatically wrong.

[mantenie]

A solution can be to replace the line with:

= I18n.t('admin.actions.delete.link', object_label: field.label)