[Explore] Allow opening the Explore screen in "guest mode"

[AkesiSeli]

On some instances, there are restrictions as to what can be displayed as a response to the "trends" APIs. If would be useful to allow users to explore other instances, retrieving contents in "guest mode" directly from another instance.

This feature is quite common in Lemmy apps (and Raccoon for Lemmy is no exception).

The idea would be to allow users (even if their Explore screen has results) to use a "⋮" button to change instance, and a button to make this feature apparent will be added in the zero results page in the Explore section.

TBD: if results are retrieved from another instance: votes/bookmarks/replies should not be allowed. This is worth investigating, and having a look at Fedilab would be useful too.