connector: implement LinkedIn connector

Signed-off-by: Pavel Borzenkov <pavel.borzenkov@gmail.com>

Do Refresh() by querying user's profile data.

Since LinkedIn doesn't provide refresh tokens at all, and the access
tokens have 60 days expiration, refresh tokens issued by Dex will fail
to update after 60 days.
Signed-off-by: Pavel Borzenkov <pavel.borzenkov@gmail.com>

connector/linkedin implements authorization strategy via LinkedIn's
OAuth2 endpoint + profile API.

It doesn't implement RefreshConnector as LinkedIn doesn't provide any
refresh token at all (https://developer.linkedin.com/docs/oauth2, Step 5
— Refresh your Access Tokens) and recommends ordinary AuthCode exchange
flow when token refresh is required.
Signed-off-by: Pavel Borzenkov <pavel.borzenkov@gmail.com>

authproxy.md: strip X-Remote-User

authproxy: update docs and set a userID

follow-up for https://github.com/coreos/dex/pull/1100

Implement the “external” connector (for Apache2 mod_auth etc.)

connector/ldap: add test for InsecureSkipVerify option

tests: add ids to elements for testing automation

travis.yml: bump golang version

connector/gitlab: Fix regexp in Link parser

fixes typo: s/suppied/supplied/

Cross clients improvement - requesting client ID always added to the audience claim

add client request timeout

storage/kubernetes: Log before registering custom resources

Logging before attempting to make any connection to Kubernetes is useful when the connection hangs and dex is killed before it can log any errors.

Add Documentation about customresourcedefinitions creation role

Replace docker-build script with multi-stage build

storage/kubernetes: Correct the OfflineSession object CRD definition

Documentation: add docs for TPR to CRD migration

storage/kubernetes: add CRD support

Updates coreos themes and icons for various providers

Makefile: error out if go files aren't correctly formatted

Noticed in #1058 that our gofmt make target isn't actually erroring
if someone commits misformatted code.