Move conversion to queue, chunk results so we don't cause php to run …

[imorland]

…out of memory

Fixes #0000

Changes proposed in this pull request:

Reviewers should focus on:

Screenshot

Confirmed

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

Required changes:

• [ ] Related Flarum core extension PR's: (Omit this section if irrelevant)