Custom message routing

[erkinalp]

@samuelscheit mentioned that we should gate this behind a permission flag, and he is right. Messing up with routes would mean drastic failures.

We should create an instance-wide permission, MANAGE_ROUTING, and a guild permission with the same name. The semantics are as follows: server-wide permissions allow you to create, alter and drop a message route targeting any channel, while per-guild permission only allows you to do so while requiring you to have either:

• Posting, storage, view all within that guild
• Posting channel is null (meaning that route is read-only) and view channel is within that guild, no constraints on storage channel

Routing rule

A routing rule shall consist of eight elements:

• Timestamp of the registration (stored as snowflake)
• Source channel
• Storage channel
• Sink channel
• Source users (user, group or role ID)
• Target users (user, group or role ID)
• Valid since timestamp (stored as snowflake)
• Valid until timestamp (stored as snowflake)

Invariants

• Valid since cannot be earlier than time of registration
• Valid until cannot be earlier than valid since
• Source user set has to have message posting permission in the source channel at the time of use
• Event type validity checks are done based on the storage channel
• Target user set has to have read messages permission in the source channel at the time of use
• Each routing rule is considered immutable once created, except for valid until timestamp, which can be set to any future time while the routing rule is still in effect

A routing rule can only be deleted in either of the following cases:

• none of the source channels are present
• none of the storage channels are present
• none of the sink channels are present
• the routing rule is not in effect AND no messages are affected by deletion of the routing rule