schema: add ttrss_prefs.access_level, bump version

schema/versions/mysql/32.sql

@@ -0,0 +1,6 @@
+alter table ttrss_prefs add column access_level integer;
+update ttrss_prefs set access_level = 0;
+alter table ttrss_prefs change access_level access_level integer not null;
+alter table ttrss_prefs alter column access_level set default 0;
+
+update ttrss_version set schema_version = 32;