[1.x] feat: allow to customize time formats through translations

[rob006]

Alternative PR for #4029, which implements concept from #4029 (comment).

It introduces 3 new keys to translate - now date formats can be adjusted by language pack or forum owner (for example through Linguist extension).

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.

Required changes:

• Related documentation PR: (Remove if irrelevant)
• Related core extension PRs: (Remove if irrelevant)

[imorland]

This makes sense to me, however I'll admit that I'm not overly familiar with different date formats, etc.

If this were included, I'd like to see an update to the Flarum documentation documenting how developers can use these changes in their customizations..

For sanity sake, let's ask @SychO9 to also take a look at this