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ARROW-7146: [R][CI] Various fixes and speedups for the R docker-compose setup

There's a lot of little things in here that have the effect of cutting 9 minutes off the build time and fixing a few bugs.

* Move R package dependency installation into the Dockerfile for better caching
* Sets MAKEFLAGS to the actual number of cores, not hard-coded to 8 (if this works out well I'll do the same to fix https://issues.apache.org/jira/browse/ARROW-6957)
* Removes hard-coded references to `bionic` so that R installation will work appropriately on other Ubuntu versions; also makes note of a dependency between OS version and availability of R versions for those who might try to run an older R version.
* Installs a lighter version of Romain's `decor` package, which cuts out dozens of unnecessary dependencies. I may remove this altogether, or at least flag it to be off on CI, because we don't need it there, it's just for developers.
* Skips vignette checks, which thereby avoid compiling and installing the R package an extra time (the check is pointless for us because there's no R code in the vignette)
* Removes an unnecessary `R CMD INSTALL`, which compiled the R bindings again (i.e., previously we compiled the R bindings 3 times, now we're down to 1)
* Turns off a check for valid URLs, saving about 30s
* Turns on more verbose error printing when there's a failure, rather than the default truncation at 13 lines.
* Uses the `rcmdcheck` package so we can fail the build on check Warnings (which cause CRAN failures). This was default on Travis.
* Adds some missing UTF-8 configuration to the dockerfile to fix some check Warnings
* Fixes some bad documentation crossreferences from #5859 that were causing other Warnings
* Correctly passes `-Werror` into the docker environment, which #5940 tried to add
* Restore the `TEST_R_WITH_ARROW` env var (lost in the move from Travis-CI) to make sure we're not just skipping all the tests

Closes #5967 from nealrichardson/tune-r-docker and squashes the following commits:

2d691aa6c <Neal Richardson> Feedback from @kou; also add env vars to appveyor job
64994c9e5 <Neal Richardson> Make sure we're testing with libarrow
9c10ca957 <Neal Richardson> Fix missing doc crossreferences from #5859
d0b07d7db <Neal Richardson> Move -Werror setting so that it works (and suppress a NOTE about it)
4e9bbe838 <Neal Richardson> Don't hard-code bionic
48091905c <Neal Richardson> Fix UTF-8 issues in R docker container
798522d7b <Neal Richardson> Oops.
d068c4fac <Neal Richardson> Restore flight off flag
26919760e <Neal Richardson> Move R dependency installation to the dockerfile. Use rcmdcheck package so we can fail the build on warnings

Authored-by: Neal Richardson <neal.p.richardson@gmail.com>
Signed-off-by: Sutou Kouhei <kou@clear-code.com>