The "Branch Size" error message and description wrongly implies that the size of all branches can be less then 100

[Softvision-RemusDranca]

[Affected versions]:

• Firefox Release 64.0.2 and above
• Chrome 71.0.3578.98
• Microsoft Edge 42.17134.1.0
• Safari 12.02

[Affected Platforms]:

• All Windows
• All Mac
• All Linux

[Prerequisites]:

• Have a new Firefox profile created.
• Have an experiment created on the "https://experimenter.stage.mozaws.net/" website with only one branch, "Control Branch".
• You are logged in your LDAP account.

[Steps to reproduce]:

1. Open the browser with the profile from prerequisites and navigate to "https://experimenter.stage.mozaws.net/".
2. Open the experiment from prerequisites and click the "Edit" button from "Branches" section.
3. Insert a lower branch size then the existing one in the "Control Branch/Branch Size" section (eg."25").
4. Scroll to the bottom of the page and click the "Save Draft" button.
5. Refocus the "Control Branch" and observe the error message displayed.
6. Click the "Help" button under the "Branch Size" section and observe the description displayed.

[Expected result]:

• Step 5: The "The size of all branches must be 100" error is displayed.
• Step 6: The "..The size of all branches together must be equal to 100.." description is displayed.

[Actual result]:

• Step 5: The "The size of all branches must add up to 100" error is displayed.
• Step 6: The "..The size of all branches together must be less than or equal to 100.." description is displayed.

[Notes]:

• Attached is a screenshot of the issue:

[Softvision-RemusDranca]

I have verified this issue and is no longer reproducible on Experimenter stage "https://stage.experimenter.nonprod.dataops.mozgcp.net/" website after the v2.28.2 stage deployment. Tested on Windows 10x64, Mac 10.14 and Arch Linux 4.16.6 x64.