Add blacklisting feature for flickr in backend

Adds an API to check for a Flickr static image or photo page URL
if the NSID of the owner is present on a configurable wikipage.
The API is used in the IsUploadAllowedFromUrl hook to refuse
upload-from-URL in such cases.

Bug: 42307
Change-Id: I97f8be973e942a732b9873c3b05f2e7790940503
Refactor Flickr API calls

* extract common arguments into a helper
* use promises to unify error handling
* cut down on indentation

Change-Id: I0e93d9af9c08cf95f7d93f8bbde0b1e6cec463ce
A way to blacklist flickr users.

For list URLs, user experience will be poor - photos with a blacklisted owner
will be skipped without any notice. This is already done for photos with
an invalid license, though, so we won't be worse off than we already are.

Bug: 42307
Change-Id: I489523463e84980f16b036f423548fdf021182a1
Merge "Add blacklisting feature for flickr in backend"
Merge "Refactor Flickr API calls"
Merge "A way to blacklist flickr users."