You can unsubscribe from Firefox Monitor with the same account more than once

[obotisan]

[Affected versions]:

• Firefox 62.0
• Firefox 64.0a1

[Affected Platforms]:

• Windows 10 x64
• macOS 10.13
• Ubuntu 16.04 x64

[Steps to reproduce]:

1. Navigate to https://fx-breach-alerts.herokuapp.com/
2. Click on the Subscribe button.
3. Write a valid email address and confirm the subscription.
4. From the report email "Unsubscribe" and see the process through and close the tab.
5. Repeat step 4.

[Expected result]:

• An error that the email is already unsubscribed to Firefox Monitor should be displayed.

[Actual result]:

• The Firefox Monitor homepage is loaded.

[Regression]:

• I don't think this is a regression.

[Notes]:

• Considering the fact that these emails are already being monitored, there should be a way to notify people that their email is already unsubscribed to this service.

[groovecoder]

I'll most likely fix this and #353 together so I'm assigning this to myself.

[mheubusch]

Recommended string: This email address is not subscribed to Firefox Monitor.

[CiprianGeorgiu]

Verified fixed on 62.0, under macOS 10.13, Ubuntu 16.04 x64 and Windows 10 x64.