mrc-2281: overhaul package

[richfitz]

This was my plan for Q1 2020 :-/

• completely rewritten test suite, makes rather heavy use of mocks and we should set up a system for a few integration tests at some point. I've added GHA (including a pkgdown site)
• completely rewritten support for workers, as rrq had moved on considerably (and we forced users to an old version: Fixes use_workers = TRUE error: invalid 'path' argument #76, Fixes use_workers = TRUE error: Error in obj$rrq$queue_submit(task_ids) : attempt to apply non-function #79)
• completely rewritten provisioning process based on conan (Fixes Error in if (as.POSIXct(t_drat) > as.POSIXct(t_lib)) occuring when returning to created context #81, Fixes Force upgrade of a package #21)
• minor updates to docs, which were incorrect in a few places (Fixes Add section to docs about the importance of relative paths #74, perhaps)
• simplifications throughout such as always setting up rtools (Fixes Add rtools when stan is a dependency #63)
• drops some bad ideas such as the helpful installation script and SQL backends (Fixes install#extras not installing required package RPostgreSQL #51)
• more vignetes! (Fixes Add example of pinning to a branch in provisioning #75, Fixes workers docs still mentions unique_value #34)
• basic reference docs for everything

Depends on a bunch of stuff:

• Overhaul to change provisioning/lib approach context#18
• Basic updates to work with new context queuer#28
• Prototype conan#1
• Re-add support for enqueue_bulk rrq#37

[weshinsley]

For a future PR, I'll fiddle through a couple of Windows tests that pass on GH, but fail locally for not obvious reasons so far. Nothing important. The vignettes: instructions seem clear to me, and following them worked exactly as described, so I think we are good to go!