Make the max_version migration reversible

This version of the down function won't necessariliy 100% replicate the
data that was dropped, but it'll get pretty darn close and probably
close enough for our purposes if we end up actually having to roll this
back.

Author: sgrif

src/bin/migrate.rs

@@ -871,8 +871,15 @@ fn migrations() -> Vec<Migration> {
         Migration::new(20170305123234, |tx| {
             tx.execute("ALTER TABLE crates DROP COLUMN max_version", &[])?;
             Ok(())
-        }, |_| {
-            panic!("Unreversible migration")
+        }, |tx| {
+            tx.execute("ALTER TABLE crates ADD COLUMN max_version VARCHAR NOT NULL DEFAULT '0.0.0'", &[])?;
+            tx.execute("UPDATE crates SET max_version = COALESCE((
+                SELECT num FROM VERSIONS
+                 WHERE num SIMILAR TO '[0-9]+\\.[0-9]+\\.[0-9]+'
+              ORDER BY split_part(num, '.', 1)::int, split_part(num, '.', 2)::int, split_part(num, '.', 3)::int
+              LIMIT 1
+            ), '0.0.0')", &[])?;
+            Ok(())
         }),
     ];
     // NOTE: Generate a new id via `date +"%Y%m%d%H%M%S"`