refactor server.py for using Blueprint

[JanEisermann]

Purpose of PR?:

Fixes #2081

Does this PR introduce a breaking change?
included Blueprint, fixed test_load_no_file

If the changes in this PR are manually verified, list down the scenarios covered::
tests succeed, not manually verified

Additional information for reviewer? :
Mention if this PR is part of any design or a continuation of previous PRs

Does this PR results in some Documentation changes?
If yes, include the list of Documentation changes

Checklist:

• Bug fix. Fixes #
• New feature (Non-API breaking changes that adds functionality)
• PR Title follows the convention of <type>: <subject>
• Commit has unit tests

[ReimarBauer]

That's good progress. We can already see it's becoming much more readable. You need to rearrange a few things.

The blueprints should contain as few additional functions as possible.

Especially not ones that need to be imported from other blueprints.

see comments