pull-requests/new with wrong capitalisation of account name redirects and discards requests params (BB-15777)

e.g. If I go to https://bitbucket.org/Foo/bar/pull-requests/new?source=wibble yet the actual account name is "foo", I will be redirected to https://bitbucket.org/foo/bar/pull-requests/new with the request parameter stripped, meaning my intention to create a PR from a certain branch is discarded, and in fact the user sometimes doesn't notice this happened and creates a PR from the wrong branch