Allow Auth messages to have detailed descriptions and default values, then give "Email Login" both

Summary:
Depends on D20664. Ref T13343. There's a reasonable value for the default "Email Login" auth message (generic "you reset your password" text) that installs may reasonably want to replace. Add support for a default value.

Also, since it isn't completely obvious where this message shows up, add support for an extended description and explain what's going on in more detail.

Test Plan:

• Viewed message detail page, saw more detailed information.
• Sent mail (got default), overrode message and sent mail (got custom message), deleted message (got default again).

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13343

Differential Revision: https://secure.phabricator.com/D20665