Limit number of gatherings on Vihko documents

[mikkohei13]

Number of gatherings on Vihko documents should be limited, so that users cannot (on purpose or accidentally) create documents with so many gatherings, that Vihko UI and/or ETL process cannot handle them.

• Limit on frontend: if document has N gatherings, don't display button or allow shortcut to add more gatherings
• Limit on backend and schema
• Limit on how many gatherings Vihko import will create. At least in the beginning, the import tool could add multiple days into one document using gathering dates. Disable that if it still exists.
• Add a note to documentation

What would be a good limit for backend/frontend? Line transect documents need to have many gatherings, but how many? E.g. this one has 76: http://tun.fi/JX.1762683