Copy/pasting links to messages

While autocomplete does handle #stream and (eventually) #stream>topic name, it would be useful to have ZT be able to copy a link to a selected message both optionally more specifically (id) and perhaps in different formats.

This could be of benefit both for:

• moving from a message context in a ZT session to a web-browser case (URL), including indirect cases such as sending a link out-of-band via eg email or pasting a link into markdown in a PR/issue (ie. outside the remit of #698)
• referring to messages better internally to ZT, ie. providing an easier way to guarantee a cross-reference is good by reading one topic, copylinking it, then composing in another location with a pastelink. Autocomplete only goes so far - where was that topic?!

The form of such a link could be:

• pm or pm+near (of no use outside of other PMs - we could warn this?)
• stream+topic or stream+topic+near

These URLs are managed by the server and already interpreted by ZT as usable jumping/narrowing opportunities (PMs will be soon).

One challenge is how to determine/specify the style/format:

• include 'near' or not? (ie. specific message or not?)
• internal link format (#stream>topic name - no near available?) vs simple absolute URL vs combined (eg. markdown)