Add ability to find users in a workspace

[andywynkoop]

Add a field to the direct messages create and channel create modal to allow users to search for other users in the workspace

• Input field on modal for entering name
• Api route to search for users by workspace THEN name
• Local state field to store added users
• modify channel post API route to join multiple users to a given channel (either DM or channel) on save
• Remove api code that joins users to a channel automatically when they send a message