connector: implement Microsoft connector

connector/microsoft implements authorization strategy via Microsoft's
OAuth2 endpoint + Graph API. It allows to choose what kind of tenants
are allowed to authenticate in Dex via Microsoft:
  * common - both personal and business/school accounts
  * organizations - only business/school accounts
  * consumers - only personal accounts
  * <tenant uuid> - only account of specific tenant

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

web/static/main.css

@@ -88,6 +88,10 @@ body {
   background-size: contain;
 }
 
+.dex-btn-icon--microsoft {
+  background-image: url(../static/img/microsoft-icon.svg);
+}
+
 .dex-btn-text {
   font-weight: 600;
   line-height: 36px;