feat: system pronouns (#429)

PluralKit.Core/Database/Migrations/28.sql

@@ -0,0 +1,7 @@
+-- schema version 28
+-- system pronouns
+
+alter table systems add column pronouns text;
+alter table systems add column pronoun_privacy integer check (pronoun_privacy in (1, 2)) not null default 1;
+
+update info set schema_version = 28;