htmltools 0.5.2 release candidate

[cpsievert]

• [x] Create branch labelled rc-v0.5.2
• [x] Change the DESCRIPTION and NEWS on this branch to change the version number to the desired version for CRAN
• [x] Commit and push to the RC branch.
• [x] Make sure all upstream dependencies are accepted on CRAN.
• [x] Remove Remotes from DESCRIPTION.
• [x] Build package
• [x] Do a sanity check by installing/testing the built package.
• [x] Run R CMD check --as-cran pkg_version.tar.gz both locally and on win-builder. You can also use https://r-hub.github.io/rhub/.
• [x] Perform revdepcheck.
• [x] Submit to CRAN
• [x] If there are R CMD check warnings that are unavoidable, make sure to explain them in the comments.
• [x] Tag release
• [x] Merge tag into master
• [x] Delete RC branch
• [x] Rebuild and deploy website
• [ ] Create GitHub release, and copy NEWS items into release notes
• [ ] Bump version number