We are moving towards making all our tests type-checked. This
commit is a move in that direction.

The 'delete topic' endpoint allows for the deletion of all messages
in a stream for a given topic.

'deleteMessagesForTopic' deletes all messages (including those in
outbox) for a given topic in a stream.

The webapp offers the feature to delete all messages within a
topic for a stream. This feature is missing in the mobile app.

Add 'Delete topic' button in message action sheet.

Fixes zulip#3806.

`deleteMessagesForTopic` currently takes both a `Narrow` and a
`Message | Outbox`. However, this is redundant; all the relevant
information can be obtained from either one.

Furthermore, neither is really necessary: we can just ask the caller
to extract the data and hand it to us. Since the caller already knows
that it's operating on a `stream` message, it's actually less trouble
than constructing the `Narrow`.

Outbox messages of type `private` will not have a comparable
`.display_recipient`, and will have no `.subject` at all.

While the comparisons currently performed are harmless and will, in
this case, yield the right result, it's cleaner conceptually (and
slightly safer when refactoring) to actually check the type first.