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)

webapp/i18n/en.json

@@ -1,4 +1,10 @@
 {
+  "multiselect.go": "Go",
+  "multiselect.instructions": "Use up/down arrows to navigate and enter to select",
+  "multiselect.placeholder": "Search and add members",
+  "multiselect.numRemaining": "You can add {num, number} more",
+  "multiselect.numPeopleRemaining": "You can add {num, number} more {num, plural, =0 {people} one {person} other {people}}. ",
+  "filtered_channels_list.count": "{count} {count, plural, =0 {0 channels} one {channel} other {channels}}",
   "about.close": "Close",
   "about.copyright": "Copyright 2016 Mattermost, Inc. All rights reserved",
   "about.database": "Database:",
@@ -1638,6 +1644,8 @@
   "more_channels.prev": "Previous",
   "more_channels.title": "More Channels",
   "more_direct_channels.close": "Close",
+  "more_direct_channels.new_convo_note": "This will start a new conversation. If you’re adding a lot of people, consider creating a private group instead.",
+  "more_direct_channels.new_convo_note.full": "You’ve reached the maximum number of people for this conversation. Consider creating a private group instead.",
   "more_direct_channels.message": "Message",
   "more_direct_channels.title": "Direct Messages",
   "msg_typing.areTyping": "{users} and {last} are typing...",