branding: move logo into database

initially, i tried to put this in the current value field but that
is TEXT and has a size limit of 64K. TEXT also stores things with
character encoding, so we have to stash it as base64

migrations/schema.sql

@@ -106,6 +106,7 @@ CREATE TABLE IF NOT EXISTS appPortBindings(
 CREATE TABLE IF NOT EXISTS settings(
     name VARCHAR(128) NOT NULL UNIQUE,
     value TEXT,
+    valueBlob MEDIUMBLOB,
     PRIMARY KEY(name));
 
 CREATE TABLE IF NOT EXISTS appAddonConfigs(