Allow use of e from Message information popup

[neiljp]

In the web app, i e shows one of the following in the drop-down menu following pressing i:

• View message source
• Edit message

This depends upon the editing rights of the user, which can vary with time.

Within ZT, users can already press e on a given message to edit it, if they are able.
However, coming from the web app, i e would give hotkey parity/compatibility.

For ZT, we currently use r for allowing showing the "raw" message, ie. i r, which shows the scrollable markdown source in a popup.

Proposal:

• If the user can edit the message, show this as a separate action under 'Viewing Actions' (possibly renamed to 'Actions'?); this would close the popup and go to the compose/edit state.
• if the user cannot edit the message, include e under the r list of keys; this should then trigger the same action as r.

There are various factors motivating this approach:

• We don't currently support simply showing a 'raw' message (ie. in markdown) in the compose box, if we can't edit it - and similarly the web app shows messages in the message feed instead
• Our r view gives the greatest scope for showing more of a message in a scrollable state, so if it's not editable, or if the user just wants to view the message, this is cleaner

Implementing this will depend upon a reliable 'can user edit topic' method, which is close to readiness via #1235.

[zulipbot]

Hello @zulip/server-hotkeys members, this issue was labeled with the "area: hotkeys" label, so you may want to check it out!

[rsashank]

@zulipbot claim

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