Central: harmonize trailing slash for WordCamp website URL

[casiepa]

Using ​https://central.wordcamp.org/wp-json/wp/v2/wordcamps?per_page=100 it seems that the URLs sometimes have a trailing slash and sometimes not.
Remove trailing slash for all (or set it, whatever is the current standard) would be preferred to harmonize:

"URL":"https:\/\/2018.islamabad.wordcamp.org\/"
"URL":"https:\/\/2019.boise.wordcamp.org"

Probably not only in the API, but better directly in the backend of Central.

[iandunn]

​3ea4ae7a fixes this for new sites, but old sites would still need to be changed.

[dd32]

This ticket has been moved to GitHub ​https://github.com/WordPress/wordcamp.org/issues/618