Replace $app->url() with url-generated link to index

[askvortsov1]

The url method of Container was removed in #2142. Works locally.

[clarkwinkelmann]

I think everywhere else we make a link to the homepage we use ->to('forum')->base() https://github.com/flarum/core/blob/df8f73bd3d656e073dcfff5bddd8e7692ca19162/src/Api/Serializer/ForumSerializer.php#L67

Or an alternative could be to use the default named route https://github.com/flarum/core/blob/2431df5602e218d77388cec0019d13a473912be0/src/Forum/ForumServiceProvider.php#L207

Using index would create a link to /all when a different homepage has been selected I believe.

[askvortsov1]

Good catch, thanks!

[franzliedke]

Removed this from the milestone as this is just a regression from another change and not relevant for the changelog or anything like that, IMO. I hope you don't mind.

[askvortsov1]

Thanks! I only added it so that we don't forget to fix it before release. The milestone will need a comb-over before release anyways