Allow configuration of a minimum password length, unify password reset
interfaces

Summary:

• We have a hard-coded minimum length of 3 right now (and 1 in the other

interface), which is sort of silly.

• Provide a more reasonable default, and allow it to be configured.
• We have two password reset interfaces, one of which no longer actually

requires you to verify you own the account. This is more than a bit derp.

• Merge the interfaces into one, using either an email token or the account's

current password to let you change the password.

Test Plan:

• Reset password on an account.
• Changed password on an account.
• Created a new account, logged in, set the password.
• Tried to set a too-short password, got an error.

Reviewers: btrahan, jungejason, nh

Reviewed By: jungejason

CC: aran, jungejason

Maniphest Tasks: T766

Differential Revision: https://secure.phabricator.com/D1374