Add upgrade step to replace truncated custom field names in history

Function custom_field_get_id_from_name() contained code to check for
truncated custom fields names. This dates back to when the history
table's field_name column was only 32 chars long, whereas the custom
field's size is 64.

The history's field_name size has been increased to 64 since 1.1.0a4
(see issue 0008002), so the check has been removed from the code and
replaced by an upgrade step (calling new install helper function
install_update_history_long_custom_fields(), which converts legacy
history entries storing truncated custom field names by their expanded
full name.

Fixes 0014650