ensure code proposal contains a valid appup file with the right versions

[tenmoves]

Description

In the current version, code proposals are built from text/plain style:

Description: ......
Changes:
{git diff}
However, to be complete and to avoid unexpected results during the upgrade, we should require the appup instructions to be present.
Hence, we could check the impact of the code proposals and the proposal's correctness.

Fixes #873

Type of change

Please delete options that are not relevant.

• New feature (non-breaking change which adds functionality)

How Has This Been Tested?

Please describe the tests that you ran to verify your changes. Provide instructions so we can reproduce. Please also list any relevant details for your test configuration

• Unit tests

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules

[samuelmanzanera]

Thanks for the PR.
Seems great 👍

I put some remarks, I would have to test it in the meantime.

[bchamagne]

I tried to create a code proposal without and with appup and it failed and worked as expected: