feat(skins): Implement custom skin backend.

src/modules/skins/settings.py

@@ -0,0 +1,54 @@
+from meta.errors import UserInputError
+from settings.data import ModelData
+from settings.setting_types import StringSetting
+from settings.groups import SettingGroup
+
+from wards import sys_admin_iward
+from core.data import CoreData
+from gui.base import AppSkin
+from babel.translator import ctx_translator
+
+from . import babel
+
+_p = babel._p
+
+
+class GlobalSkinSettings(SettingGroup):
+    class DefaultSkin(ModelData, StringSetting):
+        setting_id = 'default_app_skin'
+        _write_ward = sys_admin_iward
+
+        _display_name = _p(
+            'botset:default_app_skin', "default_skin"
+        )
+        _desc = _p(
+            'botset:default_app_skin|desc',
+            "The skin name of the app skin to use as the global default."
+        )
+        _long_desc = _p(
+            'botset:default_app_skin|long_desc',
+            "The skin name, as given in the `skins.json` file,"
+            " of the client default interface skin."
+            " Guilds and users will be able to apply this skin"
+            "regardless of whether it is set as visible in the skin configuration."
+        )
+        _accepts = _p(
+            'botset:default_app_skin|accepts',
+            "A valid skin name as given in skins.json"
+        )
+
+        _model = CoreData.BotConfig
+        _column = CoreData.BotConfig.default_skin.name
+
+        @classmethod
+        async def _parse_string(cls, parent_id, string, **kwargs):
+            t = ctx_translator.get().t
+            if string and not AppSkin.get_skin_path(string):
+                raise UserInputError(
+                    t(_p(
+                        'botset:default_app_skin|parse|error:invalid',
+                        "Provided `{string}` is not a valid skin id!"
+                    )).format(string=string)
+                )
+            return string or None
+