Fix failing invitation when claiming second zulip issue in a different zulip repository

[abhigyank]

It seems if one is already a collaborator on one project and tries @zulipbot claim on another repo, invitations are not sent to the account for collaborating for that repo. Discussion here

[amanagr]

Relevant Chats :

• https://chat.zulip.org/#narrow/stream/general/subject/Github.20Invititations/near/493675
• https://chat.zulip.org/#narrow/stream/issues/subject/.238054/near/478886

Reproducing the error:

• The user claims and accepts his first issue in zulip/X repo.
• User claims his 2nd issue in zulip/Y repo.

Problems:

• User doesn't receives the invitation second time.
• Gets this error: Sorry, we couldn't find that repository invitation. It is possible that the invitation was revoked or you are not logged into the invited account.
• User should not get the invitation when he claims an issue for the second time. He should be directly assigned.