[hotfix] shorten expiry times for channels, hotline messages, invites

context

• keeping metadata about channels/users on our servers is convenient but puts our users at risk
• this MR looks for spots where we could shorten the amount of time we keep various forms of data and does that!

changes

shorten expiry times as follows:

• channels: 4 weeks -> 1 week
• hotline message joins: 4 weeks -> 3 days
• invites: 2 weeks -> 1 week