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.pagemodule, and removes the double "account."
I also renamed AccountForm to AccountPageForm.