Add automatic coloring of tags

Please see this as proposal on how to implement automatic/random colors
for tags while keeping the current set of hard coded colors in place (at
least in the frontend).

Bear with me as I have even less Angular knowledge than Django and just
tried to get away with as few changes as possible. :-) AIUI you want to
change to a color picking system anyways in the future, which could also
play well with this.

fixes #51

src/documents/migrations/1006_migrate_tag_colour.py

@@ -0,0 +1,70 @@
+# Generated by Django 3.1.4 on 2020-12-02 21:43
+
+from django.db import migrations, models
+
+COLOURS_OLD = {
+    1: "#a6cee3",
+    2: "#1f78b4",
+    3: "#b2df8a",
+    4: "#33a02c",
+    5: "#fb9a99",
+    6: "#e31a1c",
+    7: "#fdbf6f",
+    8: "#ff7f00",
+    9: "#cab2d6",
+    10: "#6a3d9a",
+    11: "#b15928",
+    12: "#000000",
+    13: "#cccccc",
+}
+
+
+def forward(apps, schema_editor):
+    Tag = apps.get_model('documents', 'Tag')
+
+    for tag in Tag.objects.all():
+        colour_old_id = tag.colour_old
+        rgb = COLOURS_OLD[colour_old_id]
+        tag.colour = rgb
+        tag.save()
+
+
+def reverse(apps, schema_editor):
+    Tag = apps.get_model('documents', 'Tag')
+
+    def _get_colour_id(rdb):
+        for idx, rdbx in COLOURS_OLD.items():
+            if rdbx == rdb:
+                return idx
+        # Return colour 1 if we can't match anything
+        return 1
+
+    for tag in Tag.objects.all():
+        colour_id = _get_colour_id(tag.colour)
+        tag.colour_old = colour_id
+        tag.save()
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('documents', '1005_checksums'),
+    ]
+
+    operations = [
+        migrations.RenameField(
+            model_name='tag',
+            old_name='colour',
+            new_name='colour_old',
+        ),
+        migrations.AddField(
+            model_name='tag',
+            name='colour',
+            field=models.CharField(blank=True, max_length=7),
+        ),
+        migrations.RunPython(forward, reverse),
+        migrations.RemoveField(
+            model_name='tag',
+            name='colour_old',
+        )
+    ]