Support user-entered alternate sample size on Round 1

[mcchilders]

User Story:
As an audit admin, I would like to be able to use a sample size of my choosing for Round 1, so that I can go outside the suggested options if I need to.

More detail:
Note that this issue only implements this in Round 1, not in subsequent rounds. That's by design, as the use case for alternate rounds is less clear. Right now we're giving audit admins a ton of options for the sample size, but there are still good reasons an admin might want to use a number other than what we're giving them:

• For a pilot, if the 90% sample size is small (like 35 ballots, as just happened in MI!) you might want to be able to bump that up just to give people enough practice, even though that's far more ballots than you'd actually need.

• The math is constantly evolving for these audits, so to test new math you might want to run alternate calculations outside the tool while still taking advantage of most of Arlo's features, which you can do fairly easily if we let you control the size of each round.

Requirements:

• Add a final option in the list of sample size options for Round 1 labeled "Enter your own sample size (not recommended)"
• Once that radio button is selected, a text box should appear that enables the user to enter a number (text box rather than number selector, please!)
• If the radio button is deselected, the data entered should be cleared and the textbox should disappear
• If the user does select this option, we should validate that they have a) entered a number and b) entered a number that is less than or equal to the total number of ballots
• If the user selects this option, this is the sample size we should send to the back end as the desired sample size for the round.

[mcchilders]

This is the more robust issue replacing: #112