Add list of user's observations

[JWCook]

Along with pasting URLs, this may end up being the most useful way to a select observations used for tagging.

Filters, sorting, etc. will be in a separate issue.

• Add list of user's most recent observations to Observations tab
• On startup, fetch and save all new observations since last check
• Add 'refresh' button to manually fetch any new observations
  • Disambiguate from tagging 'refresh' button; may need some rework for toolbar
• Add pagination controls
• Add basic filters:
  • date range
  • iconic taxon
  • quality grade