shortcut for `PRIVATE` command

value

as an admin private messaging other admins a lot, you'd probably want a quicker way to do so without having to type PRIVATE. let's use ~!

behavior

happy path

• when admin angela sends a message saying ~let's hold off on responding to the hotline for now or sends ~ let's hold off on responding to the hotline for now (note the space)
  • then admins bob and alex both get a message saying

[PRIVATE] 
let's hold off on responding to the hotline for now

• when alex responds by saying PRIVATE ok sounds good
  • then admins angela and bob both get a message saying

[PRIVATE]
ok sounds good

sad path

• when subscriber alice tries to send ~5pm today sounds good
  • then she gets a response saying Sorry, only admins are allowed to issue that command. Send HELP for a list of valid commands.

implementation

• add ~ to constants
• fix all tests accordingly
• update HELP text (follow format for BROADCAST ! shortcut)
• update splash site how-to guide with a note on the shortcut