This commit adds a new Localization config page for
admins, as a basic filtered site setting page similar
to Legal and Notifications. Included settings are:
* default locale
* allow user locale
* set locale from accept langauge header
* onebox locale
* display local time in user card
* discourse local dates enabled
* support mixed text direction
* unicode usernames
* allowed unicode username characters
Setting tab should be added to permalinks so admins do not need to have left `/permalinks`.
A new component called `AreaSetting` was added to avoid duplications and
simplify adding settings to other sections.
A new setting attribute is used to define the areas (separated by `|`).
In addition, endpoint `/admin/config/site_settings.json` accepts new `filter_area` data.
Followup 4aea12fdcb21216a528451c0f8803e02dff24998
In certain config areas (like About) we want to be able
to fetch specific site settings by name. In this case,
sometimes we need to be able to fetch hidden settings,
in cases where a config area is still experimental.
Splitting out a different endpoint for this purpose
allows us to be stricter with what we return for config
areas without affecting the main site settings UI, revealing
hidden settings before they are ready.
