web: base custom instance functionality

also:
- renamed processing tab in settings to "instances"
- improved override description
- prefer custom over override (and grey out the option)
- dedicated lib for all api safety warnings
- left aligned small popup with smaller icon
- ability to grey out settings category & toggle

web/i18n/en/button.json

@@ -11,5 +11,6 @@
     "import": "import",
     "continue": "continue",
     "star": "star",
-    "follow": "follow"
+    "follow": "follow",
+    "save": "save"
 }