Add "Copy link to topic" to topic action sheet

[alya]

It would be very useful to be able to link to topics on mobile. We should add a "Copy link to topic" to the topic action sheet.

Following the web app ordering ("Mute topic" before "Copy link"), we could put the option after "Mute topic".

[gnprice]

See also #2623, which is for a similar option in the message action sheet.

[jyn514]

I would love it if this were the behavior of the "share" action. Right now it just shares the text of the message without context, which isn't very useful.

[Fingel]

Implemented in #5271.

[gnprice]

I would love it if this were the behavior of the "share" action. Right now it just shares the text of the message without context, which isn't very useful.

@jyn514 I agree. For that, see #2623.

(The "link to message" there would be to the message in the context of the topic, just like the link you get in the web app when getting a link to a message.)