Fixed #68. Decline invitiation will call api to mark invitation as decline.

This issue will invalidate https://github.com/mozilla/minion-backend/issues/169.
Meanwhile, the backend needs to work on:

https://github.com/mozilla/minion-backend/issues/171

Also, the frontend has to restrict access to invite that is marked decline and accepted
from visible again. See #69.
Fixed #69. Make invite inaccessible if already accepted/declined.

This fix comes from the change in backend api due to:

https://github.com/mozilla/minion-backend/issues/172
Merge pull request #70 from yeukhon/invite

Fix #68 and #69