reintroduce network tags

[kzuberi]

Previous iterations of GeneMANIA data products included 'network tags', which were simple descriptive labels associated networks and included with their display. Since they were not well used and likely in need of updating support for these was dropped when the data processing pipeline was revised. This is a reminder issue that backend support for these tags exists and they could be reintroduced if/when needed.

To add these tags back, two tables in generic_db must be populated: TAGS.txt, and NETWORK_TAG_ASSOC.txt. These tables are currently empty, and their format is documented in the GenericDb wiki page. They simply provide a vocabulary of tags and a way to associate tags to networks by id. Once populated, they will be automatically included in the lucene index and available to the website.

The previous tagging implementation required extracting MeSH terms via pubmed, and then mapping them to a smaller set of manually curated tag names.