FEATURE: Let users select flair (#13587)

User flair was given by user's primary group. This PR separates the
two, adds a new field to the user model for flair group ID and users
can select their flair from user preferences now.

spec/requests/api/schemas/json/topic_create_response.json

@@ -79,19 +79,25 @@
         "null"
       ]
     },
-    "primary_group_flair_url": {
+    "flair_name": {
       "type": [
         "string",
         "null"
       ]
     },
-    "primary_group_flair_bg_color": {
+    "flair_url": {
       "type": [
         "string",
         "null"
       ]
     },
-    "primary_group_flair_color": {
+    "flair_bg_color": {
+      "type": [
+        "string",
+        "null"
+      ]
+    },
+    "flair_color": {
       "type": [
         "string",
         "null"
@@ -219,9 +225,10 @@
     "topic_slug",
     "display_username",
     "primary_group_name",
-    "primary_group_flair_url",
-    "primary_group_flair_bg_color",
-    "primary_group_flair_color",
+    "flair_name",
+    "flair_url",
+    "flair_bg_color",
+    "flair_color",
     "version",
     "can_edit",
     "can_delete",