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-mysql.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 <= 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(ADDDATE(e2.date,1)) from eps e2 where e2.id = fe.id-1) as next_date,
-   -         DATEDIFF((SELECT ADDDATE(e3.date,1) from eps e3 where e3.id = fe.id-1),now()) AS delta_days
-   -     FROM first_empty AS fe
-   -     LEFT JOIN eps e1 ON fe.id = e1.id
-   -     LIMIT 1
-   - '
-   - %-->
-   -->
 <!--% query_latest_episodes = '
 	WITH comment_tallies AS (
 		SELECT