[Translation] [Xliff] Support for <note>
feature #10939 [Translation] [Xliff] Support for <note> (Jérémy Derussé)

This PR was squashed before being merged into the 2.6-dev branch (closes #10939).

Discussion
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[Translation] [Xliff] Support for <note>

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #10550
| License       | MIT
| Doc PR        | n/a

The purpose of this PR, is to avoid that the automated translation update (`app/console translation:update`) remove the `note` tags from XLIFF files.

Not sure if using the metadata bag is a good thing or if I should have add a "note" property in MessageCatalogue. What do you think ?

Commits
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d18e758 [Translation] [Xliff] Support for <note>