Add Slack integration

[dracan]

When configured, the following should happen:

When starting a new Pomodoro:

• The user's Slack status icon should be changed to a tomato emoji
• The status text should become "Focus time"
• Slack notifications should be turned off

When a Pomodoro ends (whether it's stopped manually or the time finishes), the above items should go back to the defaults.

[dracan]

0bf99c9