Add schema and validate platform stats requests

Added a schema to check that the start_date and end_date arguments are
both in the date format.

app/platform_stats/platform_stats_schema.py

@@ -0,0 +1,10 @@
+platform_stats_request = {
+    "$schema": "http://json-schema.org/draft-04/schema#",
+    "description": "platform stats request schema",
+    "type": "object",
+    "title": "Platform stats request",
+    "properties": {
+        "start_date": {"type": ["string", "null"], "format": "date"},
+        "end_date": {"type": ["string", "null"], "format": "date"},
+    }
+}

app/platform_stats/rest.py

@@ -4,7 +4,9 @@ from flask import Blueprint, jsonify, request
 
 from app.dao.notifications_dao import fetch_aggregate_stats_by_date_range_for_all_services
 from app.errors import register_errors
+from app.platform_stats.platform_stats_schema import platform_stats_request
 from app.service.statistics import format_admin_stats
+from app.schema_validation import validate
 
 platform_stats_blueprint = Blueprint('platform_stats', __name__)
 
@@ -12,7 +14,10 @@ register_errors(platform_stats_blueprint)
 
 
 @platform_stats_blueprint.route('')
-def get_new_platform_stats():
+def get_platform_stats():
+    if request.args:
+        validate(request.args, platform_stats_request)
+
     # If start and end date are not set, we are expecting today's stats.
     today = str(datetime.utcnow().date())