Issue title: description attribute is not translated in blueprint 
 Issue body: I notice that `description` attribute is not translated
````
                header.seotwitterimg:
                  type: pagemediaselect
                  label: MYTHEME_POST_FEATURED_IMAGE
                  help: MYTHEME_SEO_DEFAULT_IMG
                  description: MYTHEME_SEO_DEFAULT_IMG
````
 Comments: 
 Comment 0: `description` is not used by form fields.  it does no harm adding it to the definition, but it's not used in any output of the field, so it not translated.
 Comment 1: BTW if you have a custom form field that use description in, just make sure you add the `|tu` twig filter to translate it.
 Comment 2: @rhukster thanks
But i used `description` only in the backend. maybe i misunderstand the purpose of `description`
 Comment 3: Are you talking about this bit?

```
{% if files is empty %}Add files through the page media, or by dropping them in the page folder{% endif %}
``` 

Currently that's not even in translated it's hardcoded in `pagemediaselect.html.twig`
 Comment 4: no i see that PR https://github.com/getgrav/grav-plugin-admin/pull/667
 Comment 5: i think i know why because of this `{{ field.description|raw }}` https://github.com/getgrav/grav-plugin-form/blob/develop/templates/forms/default/field.html.twig#L59 it's not a "translatable"
 Comment 6: That's actually for the frontend, but in admin this is used: https://github.com/getgrav/grav-plugin-admin/blob/develop/themes/grav/templates/forms/field.html.twig#L75 - some issue though, fixed in https://github.com/getgrav/grav-plugin-admin/commit/c837942669a08bfa8c3ae5520c989fc13ff5ef80
 Comment 7: thanks a lot @flaviocopes 
{'source': 'GithubIssues', 'repo_name_with_owner': 'getgrav/grav-plugin-admin', 'issue_id': 169147622, 'repo_stars': 397.0}