db: print better error messages for invalid input

When client secrets are not base64 encoded, print an error message
that's not a generic base64 decode error:

   client secrets must be base64 decodable. See issue #337.
   Please consider replaceing "secret" with "c2VjcmV0"

When a user file is missing a mandatory field print an error message.

	Unable to build Server: user elroy-foo is missing email field

For #400

It's part of the standard tooling and no longer lives in
golang.org/x/tools.

See https://golang.org/cl/20810

add ability to revoke refresh tokens in user API

remove passwordInfos from local connector

In --no-db mode, load passwords from the users file instead of the
connectors file. This allows us to remove the password infos field
from the local connector and stop loading them during connector
registration, a case that was causing panics when using a real
database (see #286).

Fixes #286
Closes #340

add client registration to the admin API

Documentation: remove reference to DEX_APP_REDIRECTURL

Fix response_type missing param

This commit fix problem with response_type param, which is required according to OIDC spec, when it is missing.
 At now, when connector_id url query param is not set, connector view use response_type that client request instead of default "code".

Fixes #370

*: remove build-units script

server: add db heatlh checker to server checkers

move outside dependencies to ./vendor directory

Documentation: update roadmap for 0.4 release cycle

closes #328

Fixes #365

server: url decode basic auth credentials

Fixes #336

integration: fix email case sensitivity test

add support for resend an invite email

db: email comparison should be case insensitive

Fix cross compilation