Add Zumper/PadMapper to shared credentials

[dloehr]

Overall Checklist

• I agree to the project's Developer Certificate of Origin
• The top-level JSON objects are sorted alphabetically
• There are no open pull requests for the same update

for shared-credentials.json

• There's evidence the domains are currently related (SSL certificates, DNS entries, valid links between sites, legal documents etc.)
• If using shared, the new group serves login pages on each of the included domains, and those login pages accept accounts from the others. (For example, we wouldn't use a shared association from google.co.il to google.com, because google.co.il redirects to accounts.google.com for sign in.)
• If using from and to, the new group, the from domain(s) redirect to the to domain to log in.

zumperrentals.com

Clicking log in on zumperrentals.com takes the user to zumper.com to log in:

zumperrentals.mov

zumper.com and padmapper.com

Evidence that zumper.com and padmapper.com share the same credential backend includes:

1. Attempting to create an account on padmapper.com using an email address that already has an account gives the error message, "A PadMapper/Zumper user with this email already exists"
   
2. Signing in on either site redirects through all 3 sites, so that the user will be signed into the same account on all of them:
   

[rmondello]

@dloehr This change looks sensible to me, but one of our checks failed. Can you take a look and see if you want to make a change? (Let me know if something doesn’t make sense.)

Run ruby .github/workflows/lint-scripts/websites-shared-credentials-duplicates.rb
The domain 'zumper.com' appears more than once!

[dloehr]

Thanks @rmondello. I can think of 3 possibilities, do any of them seem acceptable/preferable to you?

1. Combine the 2 rules I'm adding into just

   {
       "shared": [
           "padmapper.com",
           "zumper.com",
           "zumperrentals.com"
       ]
   },

   That seems slightly incorrect to me, since zumperrentals.com redirects to zumper.com for authentication, and users no longer enter their credentials on zumperrentals.com
2. Remove the zumperrentals.com rule altogether. zumperrentals.com only had its own sign in form (which shared a backend with zumper.com and padmapper.com) for about 4 months, so I think it's relatively unimportant to mention it here
3. Adjust the lint rule from, "A domain may only be mentioned once across to, from, and shared" to "A domain may be mentioned either (1) in from only once or (2) up to once in each of shared and to"