Bug 23895: Move installer file into the mandatory directory

Bug 23895: Move installer file into the mandatory directory

Prior to this patch we had a confusing situation, mandatory files were
in both installer/data/mysql and installer/data/mysql/mandatory
To make things clearer this patch move them all into the mandatory
subdir.
However kohastructure.sql stay in the "root" directory.

Test plan:
1. Drop and recreate your DB and use the installer to populate the DB
2. Use the merge request code from misc4dev to test the reset_all alias
3. Still with the merge request from misc4dev checkout master (you can
also try 19.11 and/or 19.05) and reset_all again
=> All the data from the SQL mandatory files must have been correctly
inserted into the DB

https://gitlab.com/koha-community/koha-misc4dev/-/issues/46

Note that this patch also remove the x flag for subtag_registry.sql

Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>