feat(statistics): support for custom date ranges

[davwheat]

Changes proposed in this pull request:

• add option to choose a custom date range for the statistics widget
• add integration tests for statistics data api requests

Reviewers should focus on:

Screenshot

Necessity

• Has the problem that is being solved here been clearly explained?
• If applicable, have various options for solving this problem been considered?
• For core PRs, does this need to be in core, or could it be in an extension?
• Are we willing to maintain this for years / potentially forever?

Confirmed

• Frontend changes: tested on a local Flarum installation.
• Backend changes: tests are green (run composer test).
• Core developer confirmed locally this works as intended.
• Tests have been added, or are not appropriate here.

[SychO9]

noticed this too, I guess like we have to always type a default state/attribute type in any core components to not hinder extensibility going forward?

[davwheat]

Yeah, we've done it in some components, but not all.

Just a "simple" class MyComponent<CustomAttrs extends BaseAttrs = BaseAttrs, CustomState extends BaseState> extends BaseComponent<CustomAttrs, CustomState> should work.