LIMS-1818: Add ability to download CSV of dispensing positions

[ndg63276]

JIRA ticket: LIMS-1818

Summary:

We should add the ability to download a CSV file of all the dispensing positions marked by Chimp or users.

Changes:

• Add a new button on the view of a plate after imaging, that says "Download Dispensing CSV"
• Use signHandler/one-time token logic to secure the download
• Add a backend function to generate the CSV - the code is pretty specific to Formulatrix imagers, but for now that's all we have
• Add code for SwisSci 3 drop plates to have wells a/c/d etc, whereas other plates just use the alphabet

To test:

• Go to eg /containers/cid/354094, check there is a Download Dispensing CSV button
• Check the file downloads with the default filename of containername_target.csv
• Check the file contains a row for each drop in the plate, even if blank
• Check the well format is eg A01a, and the positions are integers