Basic telemetry

[alexdriedger]

• Added telemetry using Posthog
• Telemetry is anonymized. All telemetry is sent from the API currently. A unique UUID is generated when the api starts that acts as the distinct id for each session
• Events that are being tracked: source/model/recipient/destination created, Transfer run completed
• Telemetry can be disabled by setting PONTOON_TELEMETRY_DISABLED to true. This is also added to the readme so people can opt out of telemetry if they want to
• Created a telemetry docs page