/commands: missing command dialog

When sending a message, show an alert dialog if a command is not
recognized, offering to either cancel or send as text.

assets/l10n/intl_en.arb

@@ -86,6 +86,18 @@
     "type": "text",
     "description": "Usage hint for the command /myroomavatar"
   },
+  "commandInvalid": "Command invalid",
+  "@commandInvalid": {
+    "type": "text"
+  },
+  "commandMissing": "{command} is not a command.",
+  "@commandMissing": {
+    "type": "text",
+    "placeholders": {
+      "command": {}
+    },
+    "description": "State that {command} is not a valid /command."
+  },
   "editRoomAliases": "Edit room aliases",
   "@editRoomAliases": {
     "type": "text",
@@ -1849,6 +1861,10 @@
     "type": "text",
     "placeholders": {}
   },
+  "sendAsText": "Send as text",
+  "@sendAsText": {
+    "type": "text"
+  },
   "sendAudio": "Send audio",
   "@sendAudio": {
     "type": "text",