GSK publications: add to trial pages

[benmeg]

Now that we've added GSK as a data source, let's think about parsing the publication citations listed using some kind of lookup (e.g. Crossref's Simple Text Query) so we can add links under 'Publications' in the 'Documents' section of a trial page, and link out to the corresponding PubMed page (or the journal if not indexed by PubMed).

Here are a few examples:

• https://www.gsk-clinicalstudyregister.com/study/08-01%20(UK08)#pub
• https://www.gsk-clinicalstudyregister.com/study/095-002#pub
• https://www.gsk-clinicalstudyregister.com/study/100151#pub (N.B. multiple entries + 'Publication Type')