Offline taxon search

[kueda]

Not being able to search for and choose a taxon for an observation while offline is annoying for, well, maybe everyone, but particularly power users. It's particularly annoying when you know the app has some local taxa that we could be searching.

On TaxonSearch,

• [ ] If offline, don't hit the API and instead search Realm Taxon records; try to match the text against the name and preferredCommonName attributes; try setting and using a Realm full-text search index; but if that's taking more than half a day, the CONTAINS operator should be fine
• [ ] NEEDS DESIGN: if offline, show a notice about being offline, something like "Showing offline results. To search for more species, try again when connected to the Internet."

Related to #1958

[abhasinat]

Here's a design for a new variant of the Fixed Search Header component.

• [ ] This version of the search header component should appear when there is no internet.