PLT-3077 Add group messaging (#5489)

* Implement server changes for group messaging

* Majority of client-side implementation

* Some server updates

* Added new React multiselect component

* Fix style issues

* Add custom renderer for options

* Fix model test

* Update ENTER functionality for multiselect control

* Remove buttons from multiselect UI control

* Updating group messaging UI (#5524)

* Move filter controls up a component level

* Scroll with arrow keys

* Updating mobile layout for multiselect (#5534)

* Fix race condition when backspacing quickly

* Hidden or new GMs show up for regular messages

* Add overriding of number remaining text

* Add UI filtering for team if config setting set

* Add icon to channel switcher and class prop to status icon

* Minor updates per feedback

* Improving group messaging UI (#5563)

* UX changes per feedback

* Update email for group messages

* UI fixes for group messaging (#5587)

* Fix missing localization string

* Add maximum users message when adding members to GM

* Fix input clearing on Android

* Updating group messaging UI (#5603)

* Updating UI for group messaging (#5604)

i18n/en.json

@@ -199,6 +199,14 @@
     "id": "api.channel.create_channel.direct_channel.app_error",
     "translation": "Must use createDirectChannel API service for direct message channel creation"
   },
+  {
+    "id": "api.channel.create_group.bad_size.app_error",
+    "translation": "Group message channels must contain at least 3 and no more than 8 users"
+  },
+  {
+    "id": "api.channel.create_group.bad_user.app_error",
+    "translation": "One of the provided users does not exist"
+  },
   {
     "id": "api.channel.create_channel.invalid_character.app_error",
     "translation": "Invalid character '__' in channel name for non-direct channel"
@@ -887,6 +895,10 @@
     "id": "api.email_batching.render_batched_post.direct_message",
     "translation": "Direct Message"
   },
+  {
+    "id": "api.email_batching.render_batched_post.group_message",
+    "translation": "Group Message"
+  },
   {
     "id": "api.email_batching.render_batched_post.go_to_post",
     "translation": "Go to Post"
@@ -2151,6 +2163,14 @@
     "id": "api.templates.post_subject_in_direct_message",
     "translation": "{{.SubjectText}} in {{.TeamDisplayName}} from {{.SenderDisplayName}} on {{.Month}} {{.Day}}, {{.Year}}"
   },
+  {
+    "id": "api.templates.channel_name.group",
+    "translation": "Group Message"
+  },
+  {
+    "id": "api.templates.post_subject_in_group_message",
+    "translation": "New Group Message from {{ .SenderDisplayName}} on {{.Month}} {{.Day}}, {{.Year}}"
+  },
   {
     "id": "api.templates.reset_body.button",
     "translation": "Reset Password"
@@ -3059,6 +3079,14 @@
     "id": "app.import.import_line.null_post.error",
     "translation": "Import data line has type \"post\" but the post object is null."
   },
+  {
+    "id": "authentication.permissions.create_group_channel.description",
+    "translation": "Ability to create new group message channels"
+  },
+  {
+    "id": "authentication.permissions.create_group_channel.name",
+    "translation": "Create Group Message"
+  },
   {
     "id": "authentication.permissions.create_team_roles.description",
     "translation": "Ability to create new teams"