Release v2.18.0

The official docker release for this release can be pulled from

    quay.io/dexidp/dex:v2.18.0

Features:

* Storage: New MySQL storage backend (#1485, @bonifaido)
* gRPC: Add reflection to gRPC API (#1512, @venezia)
* Add option to always display connector selection even if there's only one (#1505, @MarcDufresne)
* Added "connector_id" to skip straight to a connector (#1481, @LanceH)
* Allow arbitrary data to be passed to templates (#1504, @MarcDufresne)
* Gitlab: implement useLoginAsID as in GitHub connector (#1497, @bonifaido)
* Microsoft: option for group UUIDs instead of name and group whitelist (#1446, @maksd)
* gRPC: Add VerifyPassword to API (#1486, @AlbanSeurat)

Bug fixes, misc changes:

* MAINTAINERS: add @bonifaido (#1492, @srenatus)
* Update ADOPTERS.md (#1495, @pbochynski; #1494, @tanmaykm; #1493, @srenatus)
* example-app: add connector_id (#1496, @srenatus)
* Docs: fix MySQL sample query (#1498, @mkontani)
* Code quality: fix some lint issues (#1500, @srenatus)
* gRPC: fix logging in VerifyPassword (#1502, @srenatus)
* Return config validation errors in one go (#1439, @sks)
* Update all deps (#1501, @srenatus)
* Return HTTP 400 for invalid state parameter (#1490, @momokatte)
* Adjusting Makefile so that `golint` will compile (#1509, @venezia)
* Add tests for some callback handler error conditions (#1510, @momokatte)
* Add examples for recent additions to oauth2 configuration options (#1516, @tpdownes)
* Bump deps for http2 issues (#1519, @srenatus)
* Connectors: refactor filter code into a helper package (#1480, @srenatus)