feat: allow multiple children to be passed to ConfirmDocumentUnload component

[davwheat]

Changes proposed in this pull request:

Lets you pass multiple children to the ConfirmDocumentUnload component through the use of a Mithril fragment.

Reviewers should focus on:

As far as I can see, the only reason this was not done before was because of the impression that this could only be done through a wrapping DOM component rather than a fragment.

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.