Allow users to merge accounts

[ilmartyrk]

Users should have possibility to merge two accounts. We have user cases where users have accidentally created multiple accounts as they were invited using e-mail and they created an new account using ID etc. Users could possibly merge two accounts into one or we allow switching accounts and then they can manually transfer topics/groups under second account and delete the first one.

[loorm]

Myself and @tiblu gave this some thought 2-3 years ago and account merging threw up any number of complex issues. So complex, in fact, that we decided to drop it.

[anettlinno]

Triage 61. Sending this issue back to In Preperation for further discussion. We have solved many problems with previous developments, like linking eID with different emails. This has solved login issues for Estonian Users.
We agreed to implement the redesign project first and then come back to merging accounts and switching accounts issues #235.

[BeccaMelhuish]

@ilmartyrk @ssin1901 seems like a 'later' one, and connected to citizenos/citizenos-fe#721