Fix/#468 user mention fixed

[umangutkarsh]

Brief Title

The PR fixes #468

Acceptance Criteria fulfillment / Issues addressed

• The members of channel should be displayed in MembersList when mentioning users
• Member list panel should be closed when message is sent.
• 'all' and 'here' mention options should be there to mention all users of channel and all online users of channel respectively.
• UI fixes
• The mention user suggestion panel is not closed when invalid user-name is entered as is closed in rocket chat.
• Enter key should send the message with mention.
• Mention can be sent along with any message typed
• Fixed selecting the members of channel from arrow-keys
• Removed the mentionmemberStore since that became redundant as member list can be populated with useMemberStore and intializing some states.

Fixes #468

Video/Screenshots

mentions.webm

mentions-more.webm

[umangutkarsh]

Will fix the linting errors

[umangutkarsh]

Hi @abhinavkrin . Please review this.
Thanks.

[Spiral-Memory]

Great work bro @umangutkarsh

[sidmohanty11]

Awesome work @umangutkarsh !! LGTM