refactor: flarum 2.0 upgrade

[SychO9]

Upgrades the extension to be compatible with Flarum 2.0

• This repository was used to test the reliability of the Flarum 2.0 upgrade tool and reinforce it. The rest of it was updated manually and contributed for your convenience.
• I will likely be posting multiple PRs like these across other repositories, and therefore will not follow-up on any changes.
• Feel free to make changes to this branch, or close the PR and re-do the upgrade yourself if you prefer.
• At the very least, this can be used as an example to guide your process either here or in other extensions.

Note

I recommend making a new branch specifically for 2.x releases, and basing this PR off of it, as you will likely want to still support 1.x for a while longer.

Tip

Checkout the upgrade guide: https://docs.flarum.org/2.x/extend/update-2_0

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Marked as draft, but you can essentially do as you please with this.
Thanks!