feat(likes): Option to prevent users liking their own posts

[imorland]

Changes proposed in this pull request:

Introduces an option to prevent forum users liking their own posts. Default to allow, as per current behavior.

I've literally lost track of how many times I've been asked to override this, so I believe it makes sense to include the option directly in the extension 🤔

Reviewers should focus on:
I'm not 100% convinced on the translation copy. Does this wording make sense?

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)

[davwheat]

Copy seems fine to me

[SychO9]

Code looks good, haven't tested it locally yet. Could we add an integration test for this?