Remove webhook with another instance of Neighbor.ly

Include migration to drop webhook_events table.
This reverts the following commits:
be25d0aaca04f051bf42cc0ea68956b5c6a466b2
24498427552ddc66d4a949de8290d50b73381cb5
5a6e8dbf6a70e38b5184d5e00ff593dd3f68d813
d2d54fe31741b53a46dd4df71051095907d6e34f

Conflicts:
    db/structure.sql
Merge pull request #15 from neighborly/im-remove-webhooks

Remove webhook with another instance of Neighbor.ly