Search delete warnings

[shapiromatron]

Add more guards to prevent cascading delete of reference imports

1. Force user to write "delete" instead of clicking a button
2. Instead of hypothetical data loss, show exactly what will be cascade deleted if a user proceeds

Make it very clear when a user is about to remove a search that has data related to it. Instead of a hypothetical warning, show exact numbers for data removed from this deletion.

Before:

After:

In addition, add guard on PubMed imports to also not delete removed references which have related studies.

[shapiromatron]

@casey1173 ready for review. I made two big changes:

1. used a template instead of string manipulation to render the message
2. updated numbers to be exact, to show what will be deleted if a user proceeds. We're using the same calculation in the warning message is what's actually done when a user deletes.

[caseyhans]

Changed some wording slightly for personal clarity--feel free to change back if you'd like.
I also changed the comma-separated list to a semicolon-separated list since most studies have commas in the name (e.g., Butterworth, 1998, 1023693).