Cancelling Custom Bounty Requires Github URL

**Steps To Reproduce**
Steps to reproduce the behavior:
1. Create a custom bounty successfully
2. Cancel bounty

**Observed behavior**
Cancelling a custom bounty requires a github URL, but the custom bounties arent tied to github

**Expected behavior**
Cancelling custom bounty shouldnt require github related information

**Note:** This change should only take place for custom bounties.. bounties created off of github issues should remained unchanged

Start by tracing the custom bounty creation and cancellation flows, then compare their validation with the GitHub-backed bounty path. The change is done when custom bounties can be cancelled without GitHub information while GitHub-linked bounty cancellation remains unchanged.

Written by the indexing model from the issue text.