umbrella-inndev 163: Homepage updates for INN Days

[benlk]

Changes

• Changes the wording of the INN description to match that requested:

  - <h5>INN is a network of more than 200 nonprofit newsrooms across the U.S., working to  strengthen the sources of trusted news for thousands of diverse communities.</h5>
  + <h5>INN supports more than 200 nonprofit newsrooms across the U.S., working to strengthen the sources of trusted news for thousands of diverse communities.</h5>

• Updates other "200+" member counts to "250+"

• Fudges the member count on inn.org/members to "250+"

• Removes the "Thanks, Supporters!" section from the homepage

• Updates INN/inn PR template to cover what we cover in https://github.com/INN/.github/blob/master/PULL_REQUEST_TEMPLATE.md

Why

For https://github.com/INN/umbrella-inndev/issues/163, by request of Sarah and approved by Sharene

Testing/Questions

Features that this PR affects:

• the homepage

Questions that need to be answered before merging:

• [ ] Is this PR targeting the correct branch in this repository?
• [x] Is this deployed to staging?
• [ ] Did we remove the right section?
• [ ] Does the updated wording match the wording in https://secure.helpscout.net/conversation/1193187834/5680?folderId=1211651

Steps to test this PR:

1. view the homepage

[benlk]

deployed to staging: https://inndev.staging.wpengine.com/

[benlk]

Approved by Sharene and Sarah, moving to deploy to prod.