Bounty Creation - Show as Organization

**User Story:**
As a funder
I want to be able to show my Github organization as the bounty creator
So that the bounty is tied to my org and not my individual account

**Acceptance Criteria**
GIVEN I am creating a bounty
WHEN i navigate to the bounty details step
THEN there will be a field labeled show as organization
AND the user should be able to enter a github org URL
AND this field should be optional

**Product & Design Links:**
https://www.figma.com/file/8AFLrc4dx9xnMXUkWOAaAR/Existing-Bounties-Updates?node-id=196%3A3433

**Notes/Assumptions:**

Start with the bounty details step and review the linked Figma design for the intended “show as organization” field. Confirm the existing bounty-creation flow and validation, then consider the work complete when an optional GitHub organization URL can be entered and the bounty creator is displayed as that organization.

Written by the indexing model from the issue text.