Create a custom mysql backend and use it

* Add a custom MySQL database backend based on django.db.backends.mysql,
  overriding only the sql_create_table template to add
  ROW_FORMAT=DYNAMIC in all CREATE TABLE statements to support indexes
  larger than 767 characters. This requires also that the MySQL server
  is configured with:
    innodb_file_per_table = 1
    innodb_file_format    = barracuda
    innodb_large_prefix   = 1
  unless it's a recent enough version in which those are the defaults.
* Use the custom MySQL backend instead of the Django's one.

Bug: T167504
Change-Id: Ia83844d7ca3e2c9afd9640d6319e9bca5623deef

debmonitor/mysql/base.py

@@ -0,0 +1,11 @@
+from django.db.backends.mysql import base, schema
+
+
+class DatabaseSchemaEditor(schema.DatabaseSchemaEditor):
+    """Override the default MySQL database schema editor to add ROW_FORMAT=dynamic."""
+    sql_create_table = "CREATE TABLE %(table)s (%(definition)s) ROW_FORMAT=DYNAMIC"
+
+
+class DatabaseWrapper(base.DatabaseWrapper):
+    """Override the default MySQL database wrapper to use the custom schema editor class."""
+    SchemaEditorClass = DatabaseSchemaEditor

debmonitor/settings/base.py

@@ -87,7 +87,7 @@
 if DEBMONITOR_CONFIG.get('MYSQL', {}):
     DATABASES = {
         'default': {
-            'ENGINE': 'django.db.backends.mysql',
+            'ENGINE': 'debmonitor.mysql',
             'NAME': DEBMONITOR_CONFIG['MYSQL']['DB_NAME'],
             'USER': DEBMONITOR_CONFIG['MYSQL']['DB_USER'],
             'PASSWORD': DEBMONITOR_CONFIG['MYSQL']['DB_PASSWORD'],