fix: swap_vanity_urls() works predictably with unset vanity URLs [DRAFT]

[toph-allen]

Intent

When swap_vanity_url() was called on a piece of content without a vanity URL, the URL would be replaced with a randomized placeholder URL during the swap.

Fixes #360

Approach

Instead of using a placeholder URL, delete the vanity URLs and re-assign them if vanity URLs are non-null.

Perhaps a better approach would be to call set_vanity_url(content, url, force = TRUE), as this doesn't risk the (admittedly very unlikely) case that another piece of content snipes the URL before it's reassigned.

I also renamed the function and its arguments, retaining the old name as a deprecated function that calls the new function for back compat. The old function was named swap_vanity_url(from, to). This did not make clear that the operation is symmetrical and operates on two pieces of content, one or both of which can have vanity URLs that are swapped. The new function is called swap_vanity_urls(content_a, content_b).

Note

This is marked as a draft for now. I want to add proper error handling, so that in the case of a permissions error we correctly restore any changes that had been made.

Checklist

• Does this change update NEWS.md (referencing the connected issue if necessary)? NOT YET
• Does this change need documentation? Have you run devtools::document()?