connector: implement LinkedIn connector

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>

web/static/main.css

@@ -83,6 +83,11 @@ body {
   background-image: url(../static/img/saml-icon.svg);
 }
 
+.dex-btn-icon--linkedin {
+  background-image: url(../static/img/linkedin-icon.svg);
+  background-size: contain;
+}
+
 .dex-btn-text {
   font-weight: 600;
   line-height: 36px;