Issue 0010912: Move install functions to new install_helper_functions_api

This commit was created out of work towards converting the Timecard
plugin from using the datetime field to an integer field. This
conversion was carried out months ago to the MantisBT core.

An upgrade function named install_date_migrate was provided in
admin/install_functions.php and usually this isn't accessible to
plugins. Therefore install_functions.php and
install_helper_functions.php from the admin directory have been merged
together into a new install_helper_functions_api.php file. The aim of
this new API is to provide update functions that are useful for not just
the MantisBT core, but also to plugins that may want to perform updates
that are consistent with changes in the MantisBT core.

The date_migrate function should not call db_get_table because that
limits usage to core MantisBT code only. Therefore the schema has been
updated to use db_get_table, ensuring that date_migrate can be used by
plugins too.