WIP: Manage OpenPGP keys backed by WkdSrv

Storage backend for keys is WkdSrv, our own API that abstracts the Web Key Directory.

Currently only one key for the primary email address may be managed. Support for multiple keys (e.g. for aliases) might be added later.

This MR also includes a simple gitlab-ci.yml to run the (few) tests, and a change to introduce a new type of flash-message that is very red and annoying (for actual client (user) errors).