[Help Wanted] [PLT-7824] Slack-compatible webhooks: Support mentions with <@userid> and <!here>

[esethna]

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Notes: Jira ticket

Slack integrations support

• mentioning users with <@userid>
• special mentions such as <!channel> and <!here>

Slack is removing usernames ([see changelog|https://api.slack.com/changelog/2017-09-the-one-about-usernames]) and is pushing people to mention users with the userid instead.

Hence, to maintain Slack-compatibility, we should add support for integrations to

1. mention users with <@userid>

Example:

a) Make a webhook post with the following payload: {"text": "hey <@beqkgo4wzbn98kjzjgc1p5n91o>"}
b) When the message is posted, "<@beqkgo4wzbn98kjzjgc1p5n91o>" should render as a user mention, such as:

2. use special mentions for at-channel, at-here and at-all via "<!channel>", "<!here>" and "<!all>", respectively.

Example:

a) Make a webhook post with the following payload: {"text": "hey <!here>"}
b) When the message is posted, "!here@beqkgo4wzbn98kjzjgc1p5n91o" should render as "@here":

NOTE: This change should be on the server-side, and only affect webhooks and custom slash commands.

[yeoji]

Hi, I gave this a go and have opened a PR :)