fix: dropdown show results search button hidden while loading results

[SychO9]

Fixes #3400

Changes proposed in this pull request:
The link to more results resides in the DiscussionSearchSource component. Search source components are currently only rendered when all of them have finished loading results, hence why the relevant link disappears. That is unnecessary and we can directly render them, I believe this used to be the previous behavior as well.

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.