Use slug driver system properly

[askvortsov1]

Closes https://github.com/FriendsOfFlarum/sitemap/issues/30

Changes proposed in this pull request: Use slug drivers to generate resource URLs

Confirmed

• [x] Frontend changes: tested on a local Flarum installation.
• [x] Backend changes: tests are green (run composer test).

[Hari-Bonda]

version not updated to 1.0.1 at extiverse.com

https://discuss.flarum.org/d/14941-fof-sitemap/245