Rename MAX_IDENTITY_ID_LENGTH to LENGTH.

The field is already in the Identity.IdentityID class, and as the assert
demonstrates it is the exact length of an identity ID. Also re-adds
documentation that the length is that of a SSK public key hash.
Move local identities page to homepage.

This page gives feedback on which identity is logged in, and is
apparently less intensive than the statistics page to assemble. The
statistics page is also registered later so that it is lower in the
list, which makes it less prominent.