fix: User link Cannot use ::class with dynamic class name

[eddiewebb]

Fixes #25

Changes proposed in this pull request: Replaces the ::class with a get_class on the $blueprint->user

Reviewers should focus on:

Screenshot Screenshot shows email dumped to logs is working after reigstration.

Confirmed

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