add 1->2 upgrade script for mysql

schema/upgrade-1.0.7-1.0.8-mysql.sql

@@ -0,0 +1,12 @@
+begin;
+
+alter table ttrss_feeds add column purge_interval integer;
+
+update ttrss_feeds set purge_interval = 0;
+
+alter table ttrss_feeds modify column purge_interval integer not null;
+alter table ttrss_feeds alter column purge_interval set default 0;
+
+update ttrss_version set schema_version = 2;
+
+commit;