Add support for temp usernames and an interstitial to confirm username

When a user now logs in for the first time for any external auth (LDAP, JWT, Keystone, Github, Google, Dex), they will be presented with a confirmation screen that affords them the opportunity to change their Quay-assigned username.

Addresses most of the user issues around #74

data/database.py

@@ -372,6 +372,22 @@ class User(BaseModel):
 Namespace = User.alias()
 
 
+class UserPromptKind(BaseModel):
+  name = CharField(index=True)
+
+
+class UserPrompt(BaseModel):
+  user = QuayUserField(allows_robots=False, index=True)
+  kind = ForeignKeyField(UserPromptKind)
+
+  class Meta:
+    database = db
+    read_slaves = (read_slave,)
+    indexes = (
+     (('user', 'kind'), True),
+    )
+
+
 class TeamRole(BaseModel):
   name = CharField(index=True)