Moves all the external login services into a set of classes that share as much code as possible. These services are then registered on both the client and server, allowing us in the followup change to dynamically register new handlers
This commit modifies the type of the login username input field from
`text` to `email`. This is an important usability feature for mobile
devices. You can still input a username instead of an email; it just
makes the `@` key easier to get to.
Fixes#306
- Adds support for Dex as an OAuth external login provider
- Adds support for OIDC in general
- Extract out external logins on the JS side into a service
- Add a feature flag for disabling direct login
- Add support for directing to the single external login service
- Does *not* yet support the config in the superuser tool
