Make SlugManager available to blade template

[imorland]

Fixes #0000

I discovered that, it is not possible to use SlugManager within a mail template. Example use case: https://github.com/FriendsOfFlarum/subscribed/blob/be01ea2b2259733c5e59b970e8bb6172243982bc/resources/views/emails/userCreated.blade.php#L5

This template errors if the user slugger is not default.

A solution is ready, but requires this PR: FriendsOfFlarum/subscribed#15

Changes proposed in this pull request:
Make SlugManager available to blade templates

Reviewers should focus on:
I also considered adding an alias, something like flarum.slug.manager here, and resolving from that in the template, but I think the above solution is probably more dev friendly
https://github.com/flarum/core/blob/master/src/Http/HttpServiceProvider.php#L64-L66

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.

Required changes:

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

[askvortsov1]

Yep, this looks good to me! Agreed that there's no need to force demand-time resolution.