Add OpenID as authentication mechanism

[sklirg]

This PR does three main things, as explained by each of the commits' messages.

1. 6a97412 makes app setup happen in async functions, so that it is possible to execute asynchronous calls during setup. This is used for automagically discovering an OpenID provider during setup, which is simpler and more prone to change than supplying the values manually.
2. 1d10ed9 simply moves logic from a huge mess of a function to a smaller function (but probably just as messy). Also splits the logic of parsing our custom OAuth2 userinfo and the creation of a user. This has the side effect of making it possible to reuse the create-user function for other authentication methods.
3. 3a6a38b adds and configures OpenID as a new authentication mechanism for the project. Currently there is no client side change, but that can be added at a later stage. The flow is exactly the same as before (click login -> redirect to OW4 OAuth2 OpenID authorization page -> accept -> logged in [notice the change?])

Some new configuration is also required if wanting to use OpenID (either instead of or in addition to OAuth2). This configuration consists of setting the address to an OpenID-capable host, supplying the client id for a valid client on the host and a redirect uri. No need to specify where the authorization, token, refresh and logout endpoints are -- they're fetched automatically. To actually activate the OIDC client it is needed to set SDF_OIDC to true.

💡 Suggestion: Read the diff of each commit on its own, they should have self-explanatory messages and it reduces the scope of the changes.

Resolves #131

[codecov]

Codecov Report

Merging #276 into master will increase coverage by 1.96%.
The diff coverage is 94.66%.

@@            Coverage Diff             @@
##           master     #276      +/-   ##
==========================================
+ Coverage   79.09%   81.06%   +1.96%     
==========================================
  Files          22       24       +2     
  Lines         751      792      +41     
==========================================
+ Hits          594      642      +48     
+ Misses        157      150       -7

Impacted Files	Coverage Δ
server/auth/index.js	81.48% <100%> (+6.48%)	⬆️
server/utils/generateTestData.js	100% <100%> (ø)	⬆️
server/auth/user.js	100% <100%> (ø)
server/auth/providers/ow4.js	84.21% <40%> (+7.12%)	⬆️
server/auth/oidc.js	96.66% <96.66%> (ø)

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[sklirg]

I tried sooo hard to achieve the target required diff coverage, but considering the fact that we're probably to remove OAuth2 in favour of OpenID Connect, I'd rather not procrastinate more time to testing that. :-P