Feature Addition: Account Linker/Switcher && Bugfix: Enable staff to see hidden threads

[LegendBegins]

Hiding a thread is no longer permanent, preventing 1-click errors (as appears to be the original intent).

Remaining TODO: Threads still show up on the homepage in the Latest section after deletion. If mods wipe a thread, that should trigger an update.

[LegendBegins]

Major caveat: This commit requires an addition to the DB schema. Specifically, a two-column table for account alts and pre-populating that table with all user accounts. The two queries I added to schema and dev_seeds should get everything set up. Reproduced below for clarity:

CREATE TABLE alts (
id SERIAL PRIMARY KEY,
ownerId SERIAL NOT NULL
);

INSERT INTO alts (id, ownerId)
SELECT id, id
FROM users;

Additional notes: I did some onclick JS in macros.html to support the dropdown menu, and left it inline since it was pretty much a one-liner. If you want me to break it into its own function in the JS file, let me know and I'll modify it.

There's a lot of code to verify here, so feel free to ping me on Discord with any questions—I think most of it should be pretty straightforward.

I do need to double check that the alts table is updated on registration. The query was super simple and seemed to work when I pasted it into Postgres, but since I haven't messed around with the test environment to get CAPTCHAs working (or bypass the requirement) I wasn't able to verify that one piece of functionality within the environment context.

[danneu]

1. The alts table contains pointers into the users table, so we can make them foreign keys with REFERENCES.
2. Postgres has case insensitive names so the convention is to use snake case to avoid confusion: ownerId -> owner_id
3. You do it in dev_seeds.sql but we should move the original seeding of the alts table into the same migration that creates the table (aka next to the create table in schema.sql).

I think all of that put together is this:

CREATE TABLE alts (
  id         integer     PRIMARY KEY REFERENCES users(id) ON DELETE CASCADE,
  owner_id   integer     NOT NULL REFERENCES users(id) ON DELETE CASCADE,
  created_at timestamptz  NOT NULL DEFAULT NOW() -- useful for these bridge tables, esp debugging
);

-- There's already an index on primary keys but lets add one for the foreign key.
CREATE INDEX idx_alts_owner_id ON alts(owner_id);

-- Initialize with existing users
INSERT INTO alts (id, owner_id)
SELECT id, id
FROM users;