Handle single name field

[artfulrobot]

If an action is configured without "Separate fields for first & last names" ticked then the extension fails to parse webhook data.

There may have been a change at iParl relating to this, or it may be just a new use case.

[artfulrobot]

iParl appears to send lastname and surname but the docs https://iparlsetup.com/setup/help#supporterwebhook only refer to surname now. Updated code to use that.

[artfulrobot]

Note: it's still a BAD IDEA to not ask users to separate their own names into first and last

Only the user knows how to spell their name. We assume that:

• a single name is a first name
• "Bam Bam Flintstone Rubble" has first name "Bam", last name "Bam Flintstone Rubble" (but could actually be that first name is "Bam Bam" and last name is "Flintstone Rubble" or some other combination.