New Project Service - Microsoft Teams
### Description
We currently use the Slack Project Service within gitlab to post to our Slack channel (via a Slack incoming webhook) whenever anyone creates or accepts a Merge Request. However our company is strongly considering migrating from Slack to a new product called [Microsoft Teams](https://products.office.com/en-us/microsoft-teams/group-chat-software) (referred to as "Teams" from hereon) and if we *do* have to migrate we would like our MR creation/acceptance to post a message to Teams instead of Slack.
### Proposal
* Add a new Project Service called **Microsoft Teams**
* Support the same triggers as the current Slack Project Service:
* Issue
* Confidential Issue
* Merge Request
* Note
* Tag push
* Build
* Pipeline
* Wiki page
### Links / references
[Getting started with Office 365 Connectors for Microsoft Teams (preview)](https://msdn.microsoft.com/en-us/microsoft-teams/connectors?f=255&MSPPError=-2147217396) - This page includes instructions for creating webhooks in Teams and how to send a message to the webhook using a JSON payload. (There's no deeplink to that section so search on the page for **Create the webhook**)

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