Add hooks for creating account pages and forms.
This introduces AccountPagesHook and AccountPageFormsHook, which allow

extensions to add to the My Account page. It also provides documentation

on using them.
While implementing an extension, I decided to change some of the naming.

reviewboard.accounts.forms.account is now

reviewboard.accounts.forms.pages, which helps tie the forms to the pages

reviewboard.accounts.page module, and removes the double "account."

I also renamed AccountForm to AccountPageForm.

Added a test page in an extension with a form. Also added a form to the

Authentication page. Both worked.