fix: notification timestamp mismatched from other core code

[davwheat]

Changes proposed in this pull request:

When running Flarum with a customised config on a server which uses a non-UTC timezone, there is a mismatch with notifications since these are handled as UTC, while other timestamps are handled in local time. This PR changes notifications to use local time, like everything else in core.

Reviewers should focus on:

Does this present an issue for existing communities? I argue not, as noone else has noticed this until now, so a fix would have just as small of an effect as the current issue has.

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.

[BartVB]

That was quick :)