Remove the KoboUserKey-based Authentication.

Instead, the user generates the api_endpoint url to set on their device
by visiting http://.../kobo_auth/generate_auth_token.
The generated url will contain a RemoteAuthorizationToken that will be
included on all subsequent requests from the device to the kobo/
endpoints. (In contrast, the device is authenticated using a session cookie on
requests to the download endpoint).

Also use Flask.url_for to generate download urls.

cps/web.py

@@ -1254,8 +1254,6 @@ def profile():
                 current_user.password = generate_password_hash(to_save["password"])
         if "kindle_mail" in to_save and to_save["kindle_mail"] != current_user.kindle_mail:
             current_user.kindle_mail = to_save["kindle_mail"]
-        if "kobo_user_key" in to_save and to_save["kobo_user_key"]:
-                current_user.kobo_user_key_hash = generate_password_hash(to_save["kobo_user_key"])
         if to_save["email"] and to_save["email"] != current_user.email:
             if config.config_public_reg and not check_valid_domain(to_save["email"]):
                 flash(_(u"E-mail is not from valid domain"), category="error")
@@ -1299,7 +1297,7 @@ def profile():
             ub.session.commit()
         except IntegrityError:
             ub.session.rollback()
-            flash(_(u"Found an existing account for this e-mail address or Kobo UserKey."), category="error")
+            flash(_(u"Found an existing account for this e-mail address."), category="error")
             return render_title_template("user_edit.html", content=current_user, downloads=downloads,
                                          translations=translations,
                                          title=_(u"%(name)s's profile", name=current_user.nickname), page="me",