Extend description in Korp config

[anne17]

Automatically extend the description fields in the Korp config with the following infos:

• download links to the sentence sets and stats files (if they exist) or to the SBX resource page
• contact email to the corpus responsible if the corpus is protected
• maybe a link to the corpus' "corpus_info" API call and to the description of Sparv standard annotation (suggested by Sasha)

One problem with this is that some of this information lives in the sparv-sbx-metadata plugin. Having a plugin dependency in a Sparv standard module will lead to Sparv crushing if the plugin is not installed).