Add download link for the metadomain alignment

[laurensvdwiel]

The prebuild alignments of homologous human protein domains is stored in the folder 'data/metadomains'. In this folder each Pfam identifier has a folder containing the alignment with the name 'metadomain_alignments'.

This is a stockholm formatted file and could be provided for the user to download via the Domain overview (for this, add a line here: https://github.com/cmbi/metadome/blob/master/metadome/presentation/web/static/js/dashboard/visualization.js#L928)