Add validation to not allow dataset URL's that aren't an XML file

[andreaszenasidi]

**WARNING: Needs to be deployed together with https://github.com/IATI/ckanext-iati/issues/365, https://github.com/IATI/ckanext-iati/issues/390 and https://github.com/IATI/ckanext-iati/issues/352**

As a publisher, I want to see a validation error when the URL to my IATI dataset is not pointing to an XML file so that I can't publish a dataset that is not in an XML format.

Acceptance criteria

1. Functional Criteria 1.1. A logged in publisher who is trying to publish a new dataset when they enter a URL that is not pointing to an XML file should see the following validation error message "Incorrect file format. All files should be in XML format that follows the IATI Standard."

1.2. A logged in publisher who is trying to update an existing dataset when they enter a URL that is not pointing to an XML file should see the following validation error message "Incorrect file format. All files should be in XML format that follows the IATI Standard."

[cormachallinanderilinx]

code is done waiting for the dependancy 352 to be finished before deploying

[cormachallinanderilinx]

Done and on staging (https://staging.iatiregistry.org/): Create or update a dataset Add a user to a file which is NOT .csv and it will not allow you to upload (when you click the upload button)

Example .csv (see blue arrow)

Use a .csv file and upload will complete (unless there are errors in some of the other fields)

[titiolukole]

Approved! This can now go into Prod

[robredpath]

@cormachallinanderilinx did this get rolled out?

[cormachallinanderilinx]

@robredpath this was reverted because it was causing the Archiver to fail. See https://github.com/IATI/ckanext-iati/issues/400