feat(emoji): allow the user to set the cdn address

[imdong]

add cdn mirror address configuration items

Changes proposed in this pull request:

Modified extension emoji, the cdn mirror address configuration item is added to the extended emoji settings page of the management interface. This option affects the Preview address of emoji in the Post, Reply, and Reply edit box Autocomplete.

Reviewers should focus on:

Some cdn only provide version Hao 14.0.2 and do not provide a short address of 14, and if the user enters the wrong address, it cannot be displayed.

Screenshot

QA

None

Necessity

• [x] Has the problem that is being solved here been clearly explained?
• [x] If applicable, have various options for solving this problem been considered?
• [x] For core PRs, does this need to be in core, or could it be in an extension?
• [x] Are we willing to maintain this for years / potentially forever?

Confirmed

• [x] Frontend changes: tested on a local Flarum installation.
• [x] Backend changes: tests are green (run composer test).
• [x] Core developer confirmed locally this works as intended.
• [x] Tests have been added, or are not appropriate here.

[imdong]

Yes, I will adapt to version 2.x later, so should I launch a new PR?

[imdong]

Submitted to #3908, this PR is obsolete