feature: implement PostgreSQL for storage of per-server settings

[RalphORama]

Summary

As LinkFix gets added to more servers, several people have reached out requesting features that would require per-server configuration. We need to store those configurations.

Methodology

Firstly, I am going to try to avoid using ORMs as much as possible. the pg package offers the ability to execute raw queries which should be good enough for our requirements. If, in the future, the database gets more complicated, I wouldn't have an issue moving to something like Sequelize.

Secondly, I want LinkFix to store as little data as possible. I think we shouldn't create records for guilds until someone in the guild actually sets configuration options. This keeps us in the clear for crap like GDPR, plus I personally believe in data privacy myself and don't want to become a vector of attack for other people.

Database Structire

https://dbdiagram.io/d/LinkFix-658b15e289dea6279992fb74

To-Do

• [x] Define schema
• [x] Add infrastructure to docker compose
• [x] Write /configure command to get and store settings
  • [ ] Add permission checks - admins only?
• [ ] Write /forget command to wipe guild from database
  • [ ] Add permission checks - admins only?
• [ ] Have replacements check their respective fields
• [ ] Check if we need to suppress embeds on reply
• [ ] Check if we need to mention the user on reply
• [ ] Check if we need to delete the original message on reply
• [ ] Write tests for all database logic

After we move into production:

• [ ] Benchmark/optimize queries
• [ ] Check for pool leaks

[shoegaze]

Add permission checks - admins only?

It's possible to set permissions for a command using SlashCommandBuilder#setDefaultMemberPermissions(): https://discordjs.guide/slash-commands/permissions.html#member-permissions

e.g.

new SlashCommandBuilder()
  .setName(...)
  // ...
  .setDefaultMemberPermissions(PermissionFlagsBits.Administrator)

should make the command admin-only.

[shoegaze]

Should settings.guild be an FK (with REFERENCES guilds(id))? Additionally, should it have ON DELETE CASCADE so that the settings rows are deleted when /forget is run? (Or should we keep the rows so that we can restore settings on /configure ?)

Also FYI: guilds.id is a bigint while settings.guild is an integer