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