Don't add my own handle to a reply (#1254)

[zeitschlag]

Fixes #1254. This was a nice little bug. This caused it:

let mentions = status.mentions ?? []
    .filter { author?.id != $0.id }
// ...

which can be read as

let mentions = status.mentions ?? [].filter { author?.id != $0.id }

So, in case there're no mentions, we filter an empty list to not include our own handle.
The fix makes the optional mentions required, as according to the docs, it's not even optional.

On another note: We should look into what it takes to remove Status-persistence.

[kimar]

👍