add schema ver 27

schema/versions/mysql/27.sql

@@ -0,0 +1,4 @@
+alter table ttrss_feeds add column last_viewed datetime;
+alter table ttrss_feeds alter column last_viewed set default null;
+
+update ttrss_version set schema_version = 27;

schema/versions/pgsql/27.sql

@@ -0,0 +1,4 @@
+alter table ttrss_feeds add column last_viewed timestamp;
+alter table ttrss_feeds alter column last_viewed set default null;
+
+update ttrss_version set schema_version = 27;