Insert a space after the username when inserting the completion

Otherwise the user is left with "nickname_" (the cursor at the _), and
it's unclear what sort of punctuation to type there.

https://gitlab.gnome.org/World/fractal/issues/184
1 job for space-after-completion in 14 minutes and 40 seconds (queued for 20 seconds)
latest
Status Job ID Name Coverage
  Test
passed flatpak #26386

00:14:40