MM-21343: Recognize multi-line slash commands without requiring trailing space after trigger word (#13646)

* change how we are parsing the args.command

* change comment

* atttempt to write unit test

* remove quotes

* use unicode.IsSpace

* MM-21343 fix panic

* use table

* test cases

* bad request, add test for missing '/'

* mispelled err

* Update app/command.go

Co-Authored-By: Jesse Hallam <jesse.hallam@gmail.com>

* Update app/command.go

Co-Authored-By: Jesse Hallam <jesse.hallam@gmail.com>

* use a map

* use subtest and add test case of empty command

* go fmt

* do not use t.fatal

* Update app/command.go

Co-Authored-By: Bot from GolangCI <42910462+golangcibot@users.noreply.github.com>

* fix missing unicode import

Co-authored-by: wiggin77 <wiggin77@warpmail.net>
Co-authored-by: Jesse Hallam <jesse.hallam@gmail.com>
Co-authored-by: mattermod <mattermod@users.noreply.github.com>
Co-authored-by: Bot from GolangCI <42910462+golangcibot@users.noreply.github.com>

i18n/en.json

@@ -499,6 +499,10 @@
     "id": "api.command.execute_command.failed_resp.app_error",
     "translation": "Command with a trigger of '{{.Trigger}}' returned response {{.Status}}."
   },
+  {
+    "id": "api.command.execute_command.format.app_error",
+    "translation": "Command trigger word is missing the leading slash character"
+  },
   {
     "id": "api.command.execute_command.not_found.app_error",
     "translation": "Command with a trigger of '{{.Trigger}}' not found. To send a message beginning with \"/\", try adding an empty space at the beginning of the message."