Allow text-entry for sample size

[umbernhard]

Rather than force users to pick one of our radio buttons, we should allow text entry of arbitrary sample sizes. E.g. if 323 ballots is the 80% sample, and 375 ballots is the 90%, users should be able to pick 350 ballots (or anything else).

[MorganLove]

@umbernhard So we could have an 'other' or 'custom' option in the radio selection, and when it is selected enable a number input next to it (and disable it when one of the other radios are selected of course). Are there any sanity checks that would need to be applied? (Like we can't have a sample size greater than the number of ballots, etc.)

[umbernhard]

@MorganLove see #113 re: input validation

I think that sounds like a good plan but defer to @mcchilders

[MorganLove]

Closing in favor of #124