Rename draft/register to draft/account-registration

[delthas]

ircv3/ircv3-specifications#435 (comment)

[DanielOaks]

To be clear, the cap name was rolled back until a change was made, and the actual latest change that rolled the cap name to draft/account-registration was this one. We should only rename the cap once we support parsing the new <accountname> parameter of the REGISTER command -- it's not a 1-for-1 swap from the old cap name to the new one.

[slingamn]

Sorry, I somehow missed all of this discussion. Let's get this done for 2.8.