This adds an integration for Microsoft Teams. Similar to our other chat

service integrations, this sends messages to channels in Microsoft Teams when

review requests are created, updated, and reviewed.
We use Microsoft's Adaptive Card format for the messages. Unlike our other chat

integrations, with MS Teams there's unfortunately no ability to add an accent

colour to the message (typically we use yellow, green, and red accents as

a visual indicator for the tone of a message). The styling of the messages

also differ between the MS teams desktop and mobile apps, but this is a

choice by Microsoft and out of our control.
Based on work by Hailan Xu (/r/11227/).

Ran unit tests.
Manually tested on RB5, RB6 and RB7:

- Creating the integration.

- New review requests, with/without diffs and files.

- Updates to review requests.

- Reviews with/without comments, issues, ship its.

- Replies.

- Tested that text which contains Markdown characters

  gets properly escaped when being formatted in a link

  in a message.

- Tested that URLs which contain ( or ) gets properly

  escaped when being formatted in a link in a message.