Use hexdump of zipfile instead of LOAD_FILE.

LOAD_FILE does have a couple of restrictions (e.g. only loads files
from server not from client). LOAD DATA LOCAL does not work with BLOB
columns. So, generate and load hexdumps manually.
Get rid of intermediate .in files for executables.
Reduce duplicated code while generating sql for executable files.
Create judgehost user with appropriate credentials and rights.

Generate restapi.secret along with dbpassword.secret. Add judgehost
user with appropriate credentials/rights to the database.
Use teamid instead of 'domjudge' when resubmitting a contestants code.