This introduces a new table to store discord message authors.
The main reason this table is being introduce is so that we don't have
to duplicate discord message author information in the `user_messages`
table when importing discord communities (ongoing work).
In addition to the table there are also two new APIs on the messenger
persistence layer (which are later used in the import logic):
- `HasDiscordMessageAuthor`
- `SaveDiscordMessageAuthor`
Closes#2759
d0e0deac95