Normalise broadcast content before validating length

This changes the content length validation of the internal API to match
the validation of the public broadcast API[1].

This removes the length check from JSONSchema, which isn’t sophisticated
enough to deal with things like normalising newlines or handling
different encodings.

The admin app should catch these errors before they’re raised here, but
it’s best to be belt and braces.

1.7ab0403ae7/app/v2/broadcast/post_broadcast.py (L53-L63)

app/broadcast_message/rest.py

@@ -2,6 +2,7 @@ from datetime import datetime
 
 import iso8601
 from flask import Blueprint, current_app, jsonify, request
+from notifications_utils.template import BroadcastMessageTemplate
 
 from app.broadcast_message.broadcast_message_schema import (
     create_broadcast_message_schema,
@@ -116,6 +117,19 @@ def create_broadcast_message(service_id):
         reference = None
     else:
         template, content, reference = None, data['content'], data['reference']
+        temporary_template = BroadcastMessageTemplate.from_content(content)
+        if temporary_template.content_too_long:
+            raise InvalidRequest(
+                (
+                    f'Content must be '
+                    f'{temporary_template.max_content_count:,.0f} '
+                    f'characters or fewer'
+                ) + (
+                    ' (because it could not be GSM7 encoded)'
+                    if temporary_template.non_gsm_characters else ''
+                ),
+                status_code=400,
+            )
 
     broadcast_message = BroadcastMessage(
         service_id=service.id,