Allow tracking to be disabled

[wmnnd]

It should be possible to disable tracking on a per-project and per-campaign basis. The project setting should be used as a default for new campaigns.

• [ ] Add setting to project settings
• [x] Add setting to campaign settings
• [x] Don't use link tracking when sending emails in non-tracked campaigns
• [x] Update campaign stats page to show that tracking was disabled

[wmnnd]

Implemented in #152.

[wmnnd]

Implemented in #152.