[Feature] PDA group messaging

[glassofmilk]

https://github.com/goonstation/goonstation-2020/pull/13

This patch adds group messaging functionality to PDAs. It changes a PDA's assigned mailgroups to a list to let you join multiple mailgroups. Starting job mailgroups like command and security can't be left or joined, but you can use the "Create/join group" and "Leave group" buttons to manage any other mail groups, allowing for players to have simple PDA group texting. Every PDA will also start connected to a leaveable "Party Line" group that allows mass texting the station easily.

I've also added a quality of life improvement to group text responses - if you get a PDA message addressed to a group, the from line will contain not only a link to respond to the sender directly but also a link to send a response to the group.

[GORE]

Hey - can you see which party PDA groups there are on the station? This seems to be a pretty neat thing and I can already see myself using it to keep in contact with those people I trust within a round.
Just wondering if another member has to invite you to such a secret group or if you can publicly join - if all groups are publicly shown or some got settings to keep them hidden? If one might need a password to join?

I am just generally a bit confused to what extend this patch works. Pretty sweet though!

[pali6]

Merged.