Add slack notifications for datasets matched to datarequests

[edmondatto]

Members of an organization should receive slack notifications for datasets that have been matched to datarequests.

These slack notifications should bring out all the bells and whistles. Maybe include a random giphy of "victory".

Ref: #10 #11

[peterpaints]

@DavidLemayian I had a discussion with @shemogumbe and @edmondatto around how this feature could be implemented, and we came up with a couple of ideas:

• The first would be to have an Add Dataset button appear (under each datarequest in the list of datarequests) when ckanext-notify is installed. We can then match the title of the dataset created through this button to an existing datarequest title (and if a match is found, a notification sent to the user that requested the data).
• The second would be to add a dropdown to the Add Dataset form when ckanext-notify is installed, that lists all existing datarequests, and users can then select the datarequest they are answering when creating a dataset.
  Do you have any other ideas on how we could go about creating this feature? I'd like your feedback on our ideas as well, and which of them, if they are viable, you'd like to see us pursue.