[2.x] feat(core): allow to disable slidable on DiscussionListItem

[DavideIadeluca]

2.x equivalent to #4302

Fixes #0000
No straight forward way to disable the Slidable Underneath Functionality on the DiscussionListItem

Changes proposed in this pull request:
Add new slidable attribute to the DiscussionListItem which allows to cleanly disable the slidable underneath functionality on the DiscussionListItem

Reviewers should focus on:
This PR is the most surgical change to preserve backwards compatibility as much as possible.

Here is how extensions can consume those new extensibility improvements:

override(DiscussionListItem.prototype, 'isSlidableEnabled', function () {
    return false;
  });

or

<DiscussionListItem discussion={discussion} params={params} slidable={false} />

This attribute is fully optional and doesn't have or need a default value of true.

Screenshot
n.a.

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)