Refactor user email confirmation + password reset handling (#919)

Co-authored-by: kolaente <k@knt.li>
Reviewed-on: https://kolaente.dev/vikunja/api/pulls/919
Co-authored-by: konrad <konrad@kola-entertainments.de>
Co-committed-by: konrad <konrad@kola-entertainments.de>

pkg/db/fixtures/user_tokens.yml

@@ -0,0 +1,18 @@
+-
+  id: 1
+  user_id: 3
+  token: 'passwordresettesttoken'
+  kind: 1
+  created: 2021-07-12 00:00:11
+-
+  id: 2
+  user_id: 4
+  token: 'tiepiQueed8ahc7zeeFe1eveiy4Ein8osooxegiephauph2Ael'
+  kind: 2
+  created: 2021-07-12 00:00:12
+-
+  id: 3
+  user_id: 5
+  token: 'tiepiQueed8ahc7zeeFe1eveiy4Ein8osooxegiephauph2Aei'
+  kind: 2
+  created: 2021-07-12 00:00:13