Do not use h3 tag for PostUser name

[rob006]

Adjustments for https://github.com/flarum/framework/pull/3732

1. It uses class name instead of targeting h3 tag in find() call, so code should work after planned changes in Flarum 2.0.
2. I switched used tag from h3 to div - this was originally planed in core, but was postponed to 2.0 because of BC break. In case of this extension BC break should have much less impact than in core, so it may be OK to switch early to markup planned for Flarum 2.0.

Confirmed

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