Gerrit sysadmin tips for account repairs/renaming

* doc/source/gerrit.rst: Add sections to the document about
resetting Gerrit SSH usernames and cleaning up duplicate preferred
E-mail addresses. Also a section demonstrating the dreaded account
merge process for posterity, even though we do not offer to perform
this for our users.

Change-Id: I43883ef9558abbdf5051aa617def36d1fc158100
Merge "Gerrit sysadmin tips for account repairs/renaming"