Next free slot algorithm now in TT² form

templates/content-index.tpl.html: Now contains TT² code and an in-built
    query to determine the next free slot. No need for an external
    query. Old code that used to use this has been deleted.

templates/queries-index-mysql.tpl.html,
templates/queries-index-sqlite.tpl.html: Old queries called
    'query_next_available_episode' removed since no longer needed.

templates/queries-index-sqlite.tpl.html

@@ -1,35 +1,3 @@
-<!--
-   - <!--% query_next_available_episode = '
-   -     WITH RECURSIVE
-   -     min_id AS (
-   -         SELECT max(id) AS start_id FROM eps WHERE date <= date(\'now\')
-   -     ),
-   -     max_id AS (
-   -         SELECT max(id) AS end_id FROM eps
-   -     ),
-   -     cnt(x) AS (
-   -         SELECT start_id from min_id
-   -         UNION ALL
-   -         SELECT x+1 FROM cnt
-   -         WHERE x+1 <= (SELECT end_id FROM max_id)
-   -     ),
-   -     first_empty AS (
-   -         SELECT
-   -             cnt.x AS id
-   -         FROM cnt
-   -         LEFT JOIN eps e ON cnt.x = e.id
-   -         WHERE e.id IS NULL
-   -     )
-   -     SELECT
-   -         fe.id as next_id,
-   -         (SELECT date(eps.date,\'+1 day\') from eps where id = fe.id-1) as next_date,
-   -         printf(\'%i\',abs(julianday((SELECT date(eps.date,\'+1 day\') from eps where id = fe.id-1)) - julianday(\'now\') + 1)) AS delta_days
-   -     FROM first_empty AS fe
-   -     LEFT JOIN eps e ON fe.id = e.id
-   -     LIMIT 1
-   - '
-   - %-->
-   -->
 <!--% query_latest_episodes = '
 	WITH comment_tallies AS (
 		SELECT