Rework Kubernetes resource naming and set up common unit tests

[lucas-koehler]

Fixes #324

Note: This will need a rebase if the formatting PR #325 is merged first. Otherwise, the touched Java files need to be formatted in the PR, too.

Rework the naming of Kubernetes resources for workspaces and sessions in the common package used by operator and service.

The goal is to make resource names more readable and contain useful information. Resource names now contain a prefix, optional identifier, user, app definition and the last segment of the source CRs (session, workspace, app definition) UID.

Also configure unit tests using JUnit 5 for the common maven module and add some tests for the new name generation.

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Notes:

• Most suffixes and prefixes containing the resource type were removed because this is already given by the type.
• The new name calculation limits names to length 57 to leave 6 characters for Kubernetes to append a hash to deployment pods. This is at least 5 characters plus a dash. If there is not enough space in the resource name, the end of the pod name is replaced with this. Thus, the limit of 57 characters to avoid this.
• Changed the identifiers for the email and proxy config maps to email resp. proxy to be as short and concise as possible.

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See an example of created resources for a user bar with app definition theia-cloud-demo:

[lucas-koehler]

@sgraband I rebased the changes and shortened the workspace prefix to ws. I did not improve the algorithm to use the space fully, yet because I'm out of time. I suggest doing this in a follow up so we can merge this already.
EDIT: Created the follow up here #327

[sgraband]

Thanks! LGTM 👍