Restyle username/password login for reduced hideousness

Summary: Ref T1536. Error state is a bit gross but we need to sort that out in general.

Test Plan:
{F46549}

{F46550}

Reviewers: chad, btrahan

Reviewed By: chad

CC: aran

Maniphest Tasks: T1536

Differential Revision: https://secure.phabricator.com/D6208
Add login buttons for button logins, fix LDAP form

Summary: Ref T1536.

Test Plan:
{F46555}

{F46556}

Reviewers: chad, btrahan

Reviewed By: chad

CC: aran

Maniphest Tasks: T1536

Differential Revision: https://secure.phabricator.com/D6209
Add a blanket "will login" event

Summary:
Ref T1536. Facebook currently does a check which should be on-login in registration hooks, and this is generally a reasonable hook to provide.

The "will login" event allows listeners to reject or modify a login, or just log it or whatever.

NOTE: This doesn't cover non-web logins right now -- notably Conduit. That's presumably fine.

(This can't land for a while, it depends on about 10 uncommitted revisions.)

Test Plan: Logged out and in again.

Reviewers: wez, btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T1536

Differential Revision: https://secure.phabricator.com/D6202