DPL-916-2 PBMC donor pooling fixes

[yoldas]

Closes #

Changes proposed in this pull request

• Show the number of source plates to scan in
• Make input fields optional
• Fix missing end to form_for block

Instructions for Reviewers

[All PRs] - Confirm PR template filled
[Feature Branches] - Review code
[Production Merges to main]
    - Check story numbers included
    - Check for debug code
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[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 90.39%. Comparing base (ef951d0) to head (af6c9df).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## develop #1622 +/- ## =========================================== - Coverage 90.80% 90.39% -0.41% =========================================== Files 359 317 -42 Lines 7418 6489 -929 =========================================== - Hits 6736 5866 -870 + Misses 682 623 -59 ``` | [Flag](https://app.codecov.io/gh/sanger/limber/pull/1622/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=sanger) | Coverage Δ | | |---|---|---| | [javascript](https://app.codecov.io/gh/sanger/limber/pull/1622/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=sanger) | `?` | | | [pull_request](https://app.codecov.io/gh/sanger/limber/pull/1622/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=sanger) | `90.39% <ø> (?)` | | | [push](https://app.codecov.io/gh/sanger/limber/pull/1622/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=sanger) | `90.39% <ø> (-0.41%)` | :arrow_down: | | [ruby](https://app.codecov.io/gh/sanger/limber/pull/1622/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=sanger) | `90.39% <ø> (ø)` | | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=sanger#carryforward-flags-in-the-pull-request-comment) to find out more.

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[codeclimate[bot]]

Code Climate has analyzed commit af6c9dfc and detected 0 issues on this pull request.

The test coverage on the diff in this pull request is 100.0% (50% is the threshold).

This pull request will bring the total coverage in the repository to 90.3% (0.0% change).

View more on Code Climate.