use `@` in hotline messages and replies

context

currently, admins have to type REPLY #1311 before replying to a hotline message. this can be cumbersome and unintuitive!

for now, this new UX will not be a breaking change to REPLY #.

behavior

tldr - create a shortcut @ for REPLY #555

• octavia is an admin on a channel
• when lauren writes a hotline message to the channel saying

hey, where's the rally tonight?

• then octavia sees a message that says

[HOTLINE FROM @75] 
hey, where's the rally tonight?

• then when octavia says

@75 meet us in prospect park!

• she (and the other admins) get a notification that says:

[REPLY TO @75]
meet us in prospect park!

• and lauren sees a message saying

[PRIVATE REPLY FROM ADMINS] 
meet us in prospect park!

help text

@1312 reply message
-> sends a private response to @1312