Enable some form of Slack notifications parsing

[wuub]

Hi,

right now mentioning slack users or teams is pretty tedious as it has to be done with slack IDs. Think
<!subteam^ID|team> or <!@U12345|nickname> instead of normal @team or @nickname.

This is because slack does not parse messages received from webhooks (default parse=none).
I think parse=full is unnecessary, but adding link_names=1 here

alertmanager/notify/impl.go

Line 465 in 2433eeb

type slackReq struct {

should be helpful.

What do you think? Are there any reasons why this is not there or will you accept a simple PR? :)

[juliusv]

I think that makes sense. I only wonder whether it should be configurable (or always on), but probably configurable.

[zerthimon]

is this gonna be merged any time soon ?

[stuartnelson3]

Following up now, sorry for letting this slip

[KirillTemnov]

We have same issue, can you fix it?

[brian-brazil]

#912 added this.

[PaulAnnekov]

Looks like nowadays link_names: true + mentioning like @username stopped working. I tried even <@username> - no luck, it just prints this text as is. The only solution is to use user id, like <@USERID>. You can find user id using this manual https://help.workast.com/hc/en-us/articles/360027461274-How-to-find-a-Slack-user-ID.
I think this change is related to the blog post https://api.slack.com/changelog/2017-09-the-one-about-usernames.