Prepare covidcast-R for release 0.5.0

[krivard]

Includes:

• Change to the multi-signal vignette: the one-state modification to fit into anonymous request limits had pushed all the subsequent operations to an empty data frame. I dropped the filters and gave you back Texas 🤠
• Version bump
• Date bump
• Documentation build - preview

I cleared out all the old docs before building the documentation site, so there are some file removals in this PR for files which are no longer produced by pkgdown. I glanced through them and the only real suspicious one is reference/covidcast.html, but its last modification date in git is from 2 years ago, and I couldn't find anywhere in the current or candidate documentation site that actually links to it; I think all that content got moved to index ages ago.

[capnrefsmmat]

Great, I'll go ahead and proceed with CRAN submission.

[capnrefsmmat]

Okay, updated the form link and set it to extract error messages on 429 responses. If that's all, I can submit to CRAN soon.

[capnrefsmmat]

Just submitted 0.5.0 to CRAN. Hopefully it goes smoothly this time.

[capnrefsmmat]

Aaand approved:

Dear maintainer,

thanks, package covidcast_0.5.0.tar.gz is on its way to CRAN.

I'll merge this once the package appears on CRAN, so the documentation is updated accordingly.