Allow to update translation comments from UI

Just to clarify the vocabulary: I'll never talk about "user comments" for discussing about a translation here, but I'm talking about the comments that can be attached to a translation key in a source file, like this:

```
// this a comment for my translation key
my.translation.key=Some translation
```

**Is your feature request related to a problem? If so, please describe.**

I recently created #5897 because we had issues with translation key comments, we will work on it, but it would have helpful for us to be able to directly edit the translation key comments directly from the UI.

**Describe the solution you'd like**

We'd like to be able to edit the translation key comments from the UI, without having to directly edit the source/translation files. Of course this should be protected with a specific right.

**Describe alternatives you've considered**

For now the only alternative I can think about is to edit directly the files on our repository.

**Screenshots**

As shown here there's no possibility to edit the comment: