[16.0][FIX] contract: Invoice creation message translatable

[pedrobaeza]

As it was, the text to translate is not correctly extracted, so it's not translated, creating a mix of languages, as _creation_message is returning a translated sentence.

We take also the opportunity to change the way the sentence is built for having as translatable only the "by contract" words, to avoid possible errors in translations due to the placeholders.

Before:

After:

@Tecnativa

[pedrobaeza]

Fast-tracking as obvious bug:

/ocabot merge patch

[OCA-git-bot]

On my way to merge this fine PR!
Prepared branch 16.0-ocabot-merge-pr-1189-by-pedrobaeza-bump-patch, awaiting test results.

[OCA-git-bot]

@pedrobaeza The merge process could not be finalized, because command git push origin 16.0-ocabot-merge-pr-1189-by-pedrobaeza-bump-patch:16.0 failed with output:

To https://github.com/OCA/contract
 ! [rejected]          16.0-ocabot-merge-pr-1189-by-pedrobaeza-bump-patch -> 16.0 (fetch first)
error: failed to push some refs to 'https://github.com/OCA/contract'
hint: Updates were rejected because the remote contains work that you do not
hint: have locally. This is usually caused by another repository pushing to
hint: the same ref. If you want to integrate the remote changes, use
hint: 'git pull' before pushing again.
hint: See the 'Note about fast-forwards' in 'git push --help' for details.

[OCA-git-bot]

Congratulations, your PR was merged at f45e8af. Thanks a lot for contributing to OCA. ❤️