Update Django 2->3 (#2121)

* used ubuntu 20.04 as base image

* updated CI image

* Fixed indent

* updated contributing guide and changelog

* update django

* Fixed migration:
this migration contains several "duplicates" (for example, it changes the id field and then changes the tarck_id field of the related table).
Since version 3.x, Django cannot apply such migrations and throws an exception.
Also checked the auto-generated migration: it doesn't contain those double changes.
In both cases, the schemas of these tables are the same.

cvat/apps/engine/migrations/0009_auto_20180917_1424.py

@@ -102,69 +102,4 @@ class Migration(migrations.Migration):
             name='id',
             field=models.BigAutoField(primary_key=True, serialize=False),
         ),
-        migrations.AlterField(
-            model_name='objectpathattributeval',
-            name='track_id',
-            field=models.BigIntegerField(),
-        ),
-        migrations.AlterField(
-            model_name='objectpathattributeval',
-            name='track_id',
-            field=models.BigIntegerField(),
-        ),
-        migrations.AlterField(
-            model_name='trackedpoints',
-            name='track_id',
-            field=models.BigIntegerField(),
-        ),
-        migrations.AlterField(
-            model_name='trackedpolygon',
-            name='track_id',
-            field=models.BigIntegerField(),
-        ),
-        migrations.AlterField(
-            model_name='trackedpolyline',
-            name='track_id',
-            field=models.BigIntegerField(),
-        ),
-        migrations.AlterField(
-            model_name='trackedboxattributeval',
-            name='box_id',
-            field=models.BigIntegerField(),
-        ),
-        migrations.AlterField(
-            model_name='trackedpointsattributeval',
-            name='points_id',
-            field=models.BigIntegerField(),
-        ),
-        migrations.AlterField(
-            model_name='trackedpolygonattributeval',
-            name='polygon_id',
-            field=models.BigIntegerField(),
-        ),
-        migrations.AlterField(
-            model_name='trackedpolylineattributeval',
-            name='polyline_id',
-            field=models.BigIntegerField(),
-        ),
-        migrations.AlterField(
-            model_name='labeledboxattributeval',
-            name='box_id',
-            field=models.BigIntegerField(),
-        ),
-        migrations.AlterField(
-            model_name='labeledpointsattributeval',
-            name='points_id',
-            field=models.BigIntegerField(),
-        ),
-        migrations.AlterField(
-            model_name='labeledpolygonattributeval',
-            name='polygon_id',
-            field=models.BigIntegerField(),
-        ),
-        migrations.AlterField(
-            model_name='labeledpolylineattributeval',
-            name='polyline_id',
-            field=models.BigIntegerField(),
-        ),
     ]

cvat/requirements/base.txt

@@ -1,5 +1,5 @@
 click==7.1.2
-Django==2.2.13
+Django==3.1.1
 django-appconf==1.0.4
 django-auth-ldap==2.2.0
 django-cacheops==5.0.1