Background:
Based on the research and work of the Editing team, new users that have traditionally used Talk Pages with VisualEditor have a growing expectation to use @ to mention and ping other users. The Android app currently uses [[User:Username]], or {{ping|Username}} as the only way to mention a user on a talk page. Once T297912 is deployed, Android users will be able to @ mention another user to ping them, but they must remember to mention that person in their reply. Ideally, when a user hits reply, the person they are replying to user name will auto populate in the reply field for them. The premise of this is on par with the motivation of topic subscriptions (increase the likelihood of users alerting the person they are talking to that they have replied T263819).

The Task:

Implement a capability that when a user presses reply to an existing thread, the user name of the person they are replying to will auto populate.

Caveats
Ensure that if someone presses reply and it auto populate, if they want to mention an additional person, both users are able to be pinged and there is the blue link confirmation that both users exist

User Story
As a talk page user on Android replying to a message received about an edit I made, I want the person I am replying to user name pre-populated when I hit reply, to decrease my chances of forgetting to tag them and them not receiving my message resulting in me having to send another or me mistakenly thinking they ignored my message.

Designs (may need coordination with engineer)
https://www.figma.com/file/LhcZQVCPJYCAV7WtngiXfD/Talk-pages-%40mention?node-id=1%3A4