[MM-23727] Make channel validation consistent on the server (#14230)

* MM-23727 Ensure user ids not allowed in channel name:

MM-23727 Move channel name validation to model level

* MM-23727 Update wording

i18n/en.json

@@ -259,10 +259,6 @@
     "id": "api.channel.create_channel.direct_channel.app_error",
     "translation": "Must use createDirectChannel API service for direct message channel creation."
   },
-  {
-    "id": "api.channel.create_channel.invalid_character.app_error",
-    "translation": "Invalid character '__' in channel name for non-direct channel."
-  },
   {
     "id": "api.channel.create_channel.max_channel_limit.app_error",
     "translation": "Unable to create more than {{.MaxChannelsPerTeam}} channels for current team."
@@ -427,10 +423,6 @@
     "id": "api.channel.update_channel.deleted.app_error",
     "translation": "The channel has been archived or deleted."
   },
-  {
-    "id": "api.channel.update_channel.invalid_character.app_error",
-    "translation": "Invalid channel name. User ids are not permitted in channel name for non-direct message channels."
-  },
   {
     "id": "api.channel.update_channel.tried.app_error",
     "translation": "Tried to perform an invalid update of the default channel {{.Channel}}."
@@ -4510,6 +4502,10 @@
     "id": "model.channel.is_valid.id.app_error",
     "translation": "Invalid Id."
   },
+  {
+    "id": "model.channel.is_valid.name.app_error",
+    "translation": "Invalid channel name. User ids are not permitted in channel name for non-direct message channels."
+  },
   {
     "id": "model.channel.is_valid.purpose.app_error",
     "translation": "Invalid purpose."