SQL autoload

[federico-razzoli]

Currently we have an empty custom.sql file included in the repo, we check if it exists and if so we run it.

Instead, let's make a directory called sql_autoload, which contains a .keep file so that git versions it. Any other file in the directory should be excluded by gitignore. After startup, we'll load each file in sql_autoload.

Files should be loaded in alphabetical order, so one can prefix them with a number to force the execution order. (In theory ls sorts files alphabetically, so it's easy)