Multiple branches with the same slug can wrongly be set for the multi-preference-experiment and branched-addon-study actions

[Softvision-PatriciuPop]

[Affected Platforms]:

• All Windows
• All Mac
• All Linux

[Affected Versions]:

• Delivery Console v1.127.0

[Prerequisites]:

• Have access to the Stage Delivery Console.

[Steps to reproduce]:

1. Open the browser and navigate to "https://delivery-console.stage.mozaws.net/".
2. Create a new multi-preference-experiment and populate the fields with valid data.
3. Add a second branch with the same name as the first one.
4. Click the "Save" button and observe the behavior.

[Expected result]:

• The "Branch Slug" fields are highlighted and an error message is displayed.

[Actual result]:

• The recipe is saved.

[Notes]:

• This issue is also reproducible for the branched-addon-study action.
• Attached a screen recording of the issue:
  

[mythmon]

Although it would be nice for DC to reject this before submitting it to the server, it is required that the server should reject bad data independent of any UI. This is really a problem with the normandy server, not with DC.