Screen-reader accessible reactions

> On iOS, sometimes I can add a reaction when double-tapping and holding, but often times, especially if the message contains a link, that double-tap and hold is executed in the middle of the cell, and if that's the link, Safari gets opened instead. The only way to solve this is via AccessibilityCustomActions, which give the developer control over anything that can be performed on a cell. That's the VoiceOver equivalent of swiping left or right, or tpping on individual portions of a message. Some apps, like Twitter, also put links in there for simplicity instead of trying to rely on the links rotor.

Start by tracing the iOS message-cell handling for reactions and links, then review how AccessibilityCustomActions exposes actions to VoiceOver. Done means a VoiceOver user can invoke the reaction action on a message containing a link without accidentally opening that link.

Written by the indexing model from the issue text.