Fixed a few minor issues

A few minor issues have been fixed:

• The message for Sampling said "percentage" instead of "rate" that is the value we are collecting right now
• The warning was shown after the Variants section and should be after the Duration one
• The progress value wasn't being rounded in the Read view as we were doing in the List one
• According to T383659 start and end date initial values should be placeholders instead of prefilled values with the date of today

Bug: T374957, T383659