Offline mode

[JWCook]

The idea behind this would be to make naturtag more usable without a reliable internet connection. I.e., don't attempt to fetch new data from the iNat API, and just do whatever we can with cached requests, images, and observation/taxon records.

Currently it is partially usable offline, but only if everything you need is already cached/stored, and it will still attempt to periodically check for new user observations.

• Tagging: If an observation ID/URL is provided but not stored locally, tag image(s) with the ID only. When an internet connection is available, the rest of the metadata can be filled in using "refresh tags."
• Observation browsing: skip check for new user observations
• Taxonomy browsing: Only use packaged taxonomy data; don't hit API to fill in missing values
• Full-size image viewer: Show thumbnail instead? Or error message?
• Add option to preload all user observation and taxon thumbnails #385