Locale filter selector based on installed languages

The problem

On the "Translations" report, the current way to filter by locales is to type the language code in the field exactly as it was defined by the developers. This causes some issues.

First, we could have some editors/admins that might just want a quick reference into translations and might not experts in the field. Hence, it is not fair to expect them to know the language codes for all installed languages. For instance, although ubiquitous, one might not know that "de" is the language code for the German language.

The second problem is more subtle and more difficult to debug when it happens. If a project defines their WAGTAIL_CONTENT_LANGUAGES as

LANGUAGE_CODE = "en"
WAGTAIL_CONTENT_LANGUAGES = LANGUAGES = (
    ("en", gettext_lazy("English")),
    ("de", gettext_lazy("German")),
    ("es", gettext_lazy("Spanish")),
    ("fr", gettext_lazy("French")),
    ("fy-NL", gettext_lazy("Frisian")),
    ("nl", gettext_lazy("Dutch")),
    ("pl", gettext_lazy("Polish")),
    ("pt-BR", gettext_lazy("Portuguese (Brazil)")),
    ("sw", gettext_lazy("Swahili")),
)

for example, the field as it current stands will only recognise "pt-BR" as the "Portuguese (Brazil)" language, but not "pt-br".

From the ITEF wiki:

Subtags are not case-sensitive, but the specification recommends using the same case as in the Language Subtag Registry, where region subtags are UPPERCASE, script subtags are Title Case, and all other subtags are lowercase. This capitalization follows the recommendations of the underlying ISO standards.

So that the "correct-er" form would be "pt-BR", although if subtags are supposed to not be case-senstive then both cases must filter properly (which is not the currently behaviour).

The solution

This PR leverages the language codes defined in WAGTAIL_CONTENT_LANGUAGES to provide chooser fields for the locales instead of relying on having to type the proper language code. This abstracts the language code information from editors/admins with the bonus of displaying language names localized to their Wagtail admin choice.

An "All" option is defined to display translations in all locales.

Screenshots:

Using the provided test app with:

LANGUAGES = WAGTAIL_CONTENT_LANGUAGES = [
    ("en", "English"),
    ("fr", "French"),
    ("fr-CA", "French (Canada)"),
    ("es", "Spanish"),
]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (e2de395) 93.27% compared to head (dcf744e) 93.27%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #716 +/- ## ======================================= Coverage 93.27% 93.27% ======================================= Files 47 47 Lines 3908 3911 +3 Branches 579 579 ======================================= + Hits 3645 3648 +3 Misses 154 154 Partials 109 109 ``` | [Files Changed](https://app.codecov.io/gh/wagtail/wagtail-localize/pull/716?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=wagtail) | Coverage Δ | | |---|---|---| | [wagtail\_localize/views/report.py](https://app.codecov.io/gh/wagtail/wagtail-localize/pull/716?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=wagtail#diff-d2FndGFpbF9sb2NhbGl6ZS92aWV3cy9yZXBvcnQucHk=) | `100.00% <100.00%> (ø)` | |

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wagtail / wagtail-localize