Bug 7533: Add a warning to the about page if template_cache_dir is not set

Bug 7533: Add a warning to the about page if template_cache_dir is not set

We need to tell the administrators that it would be great for them to
set this config entry.

Test plan:
- Do not set template_cache_dir and confirm that you see the warning
- Set template_cache_dir and confirm that you do not see the warning

Signed-off-by: Magnus Enger <magnus@libriotech.no>
Both templates for koha-conf.xml are updated. After applying the
patach a warning was correctly displayed. After adding
template_cache_dir to koha-conf.xml and restarting memcached it
went away.

Signed-off-by: Nick Clemens <nick@bywatersolutions.com>