Add support of Slack user-group mentioning in the alert message

[ArXa1L]

Hello,
My team has a single channel for alerts across all our applications. For each application we added a separate Slack user-group (@gitlab_duty/@teamcity_duty for example) that contains one or several engineers on duty and we would like to mention these groups in the alert message.

Moira uses Slack API chat.postMessage for sending alert messages and that requires us to specify that user group by using the rather complicated syntax <!Subteam^SAZ94GDB8> (see https://api.slack.com/reference/surfaces/formatting#mentioning-groups). We can't get user-group ID from the Slack Desktop App and we have to use Slack API in order to do that.

There is another way: we can specify additional property Parse with value full in PostMessageParameters which change how Slack treats the message (see https://api.slack.com/methods/chat.postMessage#formatting). In my opinion this way is more user-friendly and the required changes are minor.

[coveralls]

Coverage decreased (-0.02%) to 75.247% when pulling 1038a91 on ArXa1L:feature/slack/parse into 85cf51d on moira-alert:master.

[ArXa1L]

According to unofficial information, the "Parse" can be deprecated and will be removed soon. Just in case, I added one more property that changes in similar way how Slack API processes the message (see https://api.slack.com/methods/chat.postMessage#arg_link_names)