Add author type on the image's informations

[flob38]

In the available informations for images, we have 2 sort of author : author of the document (the one who created the document on C2C) and the author of the image (the one who shooted it).
When displaying those info, we must use suitable labels.

[flob38]

Maybe it is only a problem of traduction.

<field-view :document="document" :field="fields.author" />
<label-value v-if="document.creator" :label="$gettext('creator')">
     <author-link :author="document.creator" />
</label-value>

[cbeauchesne]

First line : it's image author, which means the one who took the picture
Second line : it's the one who uploaded the picture on camptocamp.

We must ask community what are the best translations for them, and fix it on transifex.

I propose :

• first line : Auteur de l'image
• second line : Contributeur

Note that the first translation key (author) is hardcoded, as it comes from API. We can't change it.
The second translation key, creator, can be changed, and as it's actually missleading, must be changed. If we choose "contributeur" in french, we'll change translation key to contributor.

[cbeauchesne]

Duplicated of #590, and it's fixed by #591. Sorry @flob38, I've forgotten that you have created this issue when I opened #590