Bug 14899: DB changes

Bug 14899: DB changes

This patch applies several changes to the DB structure introduced by
the previous ES patch set:

1/ Add a search_field.label column
=> It will be easier to understand what the search_field does.
Indeed, some are not user friendly: acqdate, an, pl, ff8-23, ln, etc.
TODO later: Update the labels with correct values (at the moment
label=name)

2/ Update the foreign key search_marc_to_field.search_marc_map_id with
on delete cascase. This will permit to automatically remove the entries
in search_marc_to_field when they are deleted in search_marc_map.

3/ Remove the index_name_2 unique constraint on the search_marc_map table.
I don't understand how this could be useful, it was defined like (index_name,
marc_type, marc_field), so it means a mapping cannot be defined twice with
different values for facet, suggestible and sort.
This limitation does not seem to make sense.
Robin, please correct me if I am wrong :)

Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Jesse Weaver <jweaver@bywatersolutions.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>