Allow gd or imagick image extensions

[doxroot]

Fixes #0000

Changes proposed in this pull request:
This changes do not affect in any way image upload.
I tested with avatar uploaded.
Since the fallback is gd, the default driver should be imagick
(Reffer to src/Filesystem/FilesystemServiceProvider.php)

PHP works with both gd and imagick. I don't see point in limitations of gd.
Actually, flarum don't use gd especially, i don't see any image manipulation that imagick can't do it.

Reviewers should focus on:
Image uploads.

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)