Fix avatar-upload redirect url on Edit Public Profile  page

The avatar-upload-service accepts a 'redirect_id' where- once your image
is posted - it will return you. Unfortunately, we were defining the
field like this in our form:

<input type="hidden" name="redirect_url" value="https://profile.theguardian.com/public/edit?returnUrl=http%3A%2F%2Fwww.theguardian.com%2Fuk%3Fview%3Dmobile">

The '?returnUrl=http%3A%2F%2Fwww.theguardian.com%2Fuk%3Fview%3Dmobile' part
of that url is superfluous, and unfortunately the gu-image-upload service
has a bug in /it's/ url construction (where it adds the 'signed_data'
parameter) so it attempts to send the user to a subtly invalid url like
this:

https://profile.theguardian.com/public/edit?returnUrl=http%3A%2F%2Fwww.theguardian.com%2Fuk%3Fview%3Dmobile?signed_data=eyJ1c2VybmFtZSI6IlJvYmVydG8gVHlsZXkiLCJpbWFnZV91cGxvYWRfc3VjY2VzcyI6dHJ1ZSwidXNlcl9pZCI6IjEyNTk3Mjk3IiwicmVxdWlyZWRfaW1hZ2VfaG9zdCI6Imh0dHA6XC9cL3N0YXRpYy5ndWltLmNvLnVrIiwiaXNfc29jaWFsIjpmYWxzZX0.Xpp86s1Kz1cDLEI_q5UaEeT5kEs

Note that the 'signed_data' is prefixed with a '?' rather than an '&', so
the url is now corrupt. The easiest way to avoid this bug is just to remove
the superfluous part of our requested redirect_url, which is what this
commit does.

The 'idRequest' parameter was responsible for adding this unwanted extra
segment:

https://github.com/guardian/frontend/blob/8b51e6d0/identity/app/services/IdentityUrlBuilder.scala#L9-L25


https://jira.gutools.co.uk/browse/IDN-1774
https://profile.theguardian.com/public/edit?returnUrl=http%3A%2F%2Fwww.theguardian.com%2Fuk%3Fview%3Dmobile
Merge pull request #3960 from guardian/fix-avatar-upload-redirect-url

Fix avatar-upload redirect url on Edit Public Profile  page