Automatically add redirects for topic aliases

[hadley]

Fixes #1876

@DavisVaughan @lionel- would one of you mind double checking that this creates the right aliases for one of vctrs/tidyr/dplyr/etc?

[lionel-]

@hadley If I build the rlang site locally, then try e.g. dev/reference/tidy-dots.html (should be aliased to dyn-dots.html), I'm seeing a 404 error. Am I missing anything?

[hadley]

@lionel- hmmm, that works for me. Did you see the lines like:

Adding redirect from reference/syms.html to reference/sym.html.
Adding redirect from reference/tidy-dots.html to reference/dyn-dots.html.
Adding redirect from reference/tidyeval-data.html to reference/dot-data.html.

Also note that due because of the way pkgdown sites are designed, the redirect will always go to the live site.

[lionel-]

It works!