Deduplicate default filename template code

Summary:
In preparation for user-visible improvements to the default filenames,
let's move the corresponding code around so that it only has to be
touched in a single place, preventing strings getting out of sync with
time.

Ref T9252

Test Plan:
{nav Configure > Save} dialog page should look like before, and
{nav Save} with a fresh config should populate the filename as usual.

Reviewers: #spectacle, ngraham

Reviewed By: #spectacle, ngraham

Subscribers: ngraham

Maniphest Tasks: T9252

Differential Revision: https://phabricator.kde.org/D14588