[16.0][FW] [14.0] product_template_multi_link: improve UX by searching templates on default_code

[henrybackman]

Port of #800 from 14.0 to 16.0.

The test SavepointCase was changed to TransactionCase according to the migration guides, but no separate commit for that and amended instead

[simahawk]

/ocabot merge minor

[OCA-git-bot]

This PR looks fantastic, let's merge it!
Prepared branch 16.0-ocabot-merge-pr-983-by-simahawk-bump-minor, awaiting test results.

[OCA-git-bot]

This PR has the approved label and has been created more than 5 days ago. It should therefore be ready to merge by a maintainer (or a PSC member if the concerned addon has no declared maintainer). 🤖

[OCA-git-bot]

@simahawk your merge command was aborted due to failed check(s), which you can inspect on this commit of 16.0-ocabot-merge-pr-983-by-simahawk-bump-minor.

After fixing the problem, you can re-issue a merge command. Please refrain from merging manually as it will most probably make the target branch red.

[github-actions]

There hasn't been any activity on this pull request in the past 4 months, so it has been marked as stale and it will be closed automatically if no further activity occurs in the next 30 days.
If you want this PR to never become stale, please ask a PSC member to apply the "no stale" label.