The branch size can wrongly be saved with whole numbers larger or smaller than 100

[Softvision-RemusDranca]

[Affected versions]:

• Firefox Release 65.0.2 and above
• Chrome 72.0.3626.121

[Affected Platforms]:

• All Windows
• All Mac
• All Linux

[Prerequisites]:

• You are logged with your LDAP account on the "https://experimenter.stage.mozaws.net/" website.
• Have an experiment created on the Experimenter stage website.

[Steps to reproduce]:

1. Open the browser with the profile from prerequisites and navigate to "https://experimenter.stage.mozaws.net/".
2. Open the created experiment from prerequisites.
3. Click the "Edit" button from "Branches" section.
4. Enter a whole number larger or smaller than 100 in the "Branch Size" input field.
5. Click the "Save Draft" button and observe the behavior.

[Expected result]:

• The "! The size of all branches must add up to 100 " error message is displayed.

[Actual result]:

• The draft is saved.

[Notes]:

• This issue is also reproducible if multiple Branches are created and saved with different branch sizes. The entered numbers sum can result in values smaller or larger than 100.
• Attached is a screen recording of the issue:
  

[peterbe]

The explanation is that ExperimentVariantsPrefFormSet.clean doesn't do the same total_percentage validation that ExperimentVariantsFormSet.clean does.

[jaredlockhart]

@peterbe Oh you're too fast! I didn't see you picked this up, I already submitted a pr. I should have self assigned this faster.

[Softvision-RemusDranca]

I have verified this issue and it's no longer reproducible on the "https://experimenter.stage.mozaws.net/" website after the (v2.25.1) deployment.
Tested on Windows 10x64, MacOS 10.14 and Arch Linux 4.16.6 x64.