Support linking GitHub users by replying to bot event messages

[ASafaeirad]

Make the /link command easier to use by allowing users to reply directly to any bot event message that contains a GitHub account.

Proposed behavior

When a user replies to a bot-generated GitHub activity message with:

/link @tgusername

the bot should:

1. Check that the replied-to message was sent by the bot itself
2. Parse the replied message and extract the GitHub username/account
3. Link the extracted GitHub account to the provided Telegram username

Example

User replies to this bot message:

🌟 [ASafaeirad](https://github.com/ASafaeirad) gave star number 18 to [yt-channel-mirror](https://github.com/fullstacksjs/yt-channel-mirror).

with:

/link @S_Kill

Expected result:

GitHub user ASafaeirad has been linked to @S_Kill.