export tool should show project observation fields

[kueda]

Right now the export tool just the current user's most recent observation fields, but people who manage projects want to make sure they can select their projects observation fields, even if they personally haven't used them recently. There are a couple ways to go about this, so here are some tiers:

Minimum

• observation fields shown should include all the project observation fields for projects specified in the URL, e.g. http://www.inaturalist.org/observations/export?projects%5B%5D=zavamaniry-gasy-plants-of-madagascar
• they should be shown in a separate section. e.g. "Observation fields for PROJECT NAME"

Ideal

• add a more sophisticated project selector to the observation filter form
• changing that selector should request projects/show.json to bring down the project observation fields and show those as options in the observation fields section