Allow merging user accounts

[slel]

Please provide the ability to merge user accounts.

The need to do that might arise from various situations.

• user opens second account and later asks for a merge
• problem during a system or Django or Askbot migration
• an authentication method is removed, a password is lost

References:

• Issue 110 on the old Askbot issue tracker (2011-10-17)
• Ask Askbot question 3143: merge users? (2011-10-17)

[slel]

Requested on Ask Sage:

• Ask Sage question 53972: Uniting new and old forum account?

[evgenyfadeev]

It is possible to do by running: python manage.py merge_users <from_id> <to_id>

There could be a feature though to merge users from the UI.