Add the ability to merge accounts

[Cheshiriks]

Now the user cannot add his account from another source. It will be created as a new user.

• [x] #1291
• [x] Support external OAuth providers
  • [x] Google
  • [x] Huawei
  • [x] Gitee

Related: #1291

[nulls]

Scenario:

• User has approved account
• User logs out from this account
• User logs in using a new account (oauth)
• During creating a new account user sets user name for existed account.
• New account lefts in not approved state (or maybe in some another state)
• User logs in to old approved account and approves merging
• Profit