Order of strings in monolingual translations should match template order

### Describe the problem

I've noticed that the order of strings in translated Fluent files for Tor Browser do not match the order in the original en-US file.

Instead, it seems that the strings are ordered by when the unit was created, or the order in which strings have been translated.

E.g. if I have the original file:

```
message-1 = First
message-2 = Second
```

and on Weblate I first translate `message-2` and then translated `message-1`, the resulting translated file is:

```
message-2 = Second translated
message-1 = First translated
```

This makes it more difficult to compare the translated files with each other, or the original en-US file.

It also means that if we makes it harder to review bulk automated change to all translations: (such as [Fluent migrations](https://firefox-source-docs.mozilla.org/l10n/migrations/fluent.html) because these will arrange the strings according to the template en-US file.

### Describe the solution you would like

At least for the Fluent format, it would be nice if the order of strings could match the template's order.

### Describe alternatives you have considered

_No response_

### Screenshots

_No response_

### Additional context

_No response_

Start by tracing how Weblate exports translated Fluent files and how it obtains the template's message order; the issue does not name an implementation file or test. Done means translated Fluent output follows the en-US template order rather than translation or unit-creation order, with coverage for the demonstrated message-1/message-2 case.

Written by the indexing model from the issue text.