Release xaringanExtra 0.7.0

[gadenbuie]

Prepare for release:

• [x] git pull
• [x] npm audit
• [x] npm run build
• [x] Check current CRAN check results
• [x] Check if any deprecation processes should be advanced, as described in Gradual deprecation
• [x] Polish NEWS
• [x] devtools::build_readme()
• [x] urlchecker::url_check()
• [x] devtools::check(remote = TRUE, manual = TRUE)
• [ ] devtools::check_win_devel()
• [ ] ~rhub::check_for_cran()~
• [ ] ~revdepcheck::revdep_check(num_workers = 4)~
• [x] Update cran-comments.md
• [x] git push
• [x] Draft blog post

Submit to CRAN:

• [x] usethis::use_version('minor')
• [x] devtools::submit_cran()
• [x] Approve email

Wait for CRAN...

• [ ] Accepted :tada:
• [ ] git push
• [ ] usethis::use_github_release()
• [ ] usethis::use_dev_version()
• [ ] git push
• [ ] Finish blog post
• [ ] Tweet
• [ ] Add link to blog post in pkgdown news menu