Improve avatar merging process

[pferreir]

Right now we are checking for all the places where a user exists, by using the linkedTo dictionary. This is not only slow, but hard to maintain as well.

We could follow a different strategy:

• When two avatars are merged, link the merged avatar to the one it's been merged into, and add the former to the latter's "sub-avatar" list. This would a list of all identities that had been merged into a specific Avatar;
• The parent avatar is called instead of the sub-avatar whenever checks are to be made (or information extracted) - this could be done with a sub-class (?);

This would have the following advantages:

• Faster merging;
• No loss of data;
• "Unmerging" becomes possible without a lot of work;

[jbenito]

Beware that this should work also for favorites. We have had many cases of people using favorites that do not exist anymore because of merging.

[pferreir]

I'm moving this to 2.0 since it very frequently causes issues. The new relational backend should allow us to perform this kind of operation in a much safer way and with better performance.