New columns for moderation

 * db/migrations directory to slightly organise our migrations
   (no instrumentation is yet provided for this.  Will need to
   also update db/schema.sql when this is finalized)
 * added moderation user, timestamp, and comment fields for
   pledges and comment
 * added ishidden field for pledges, to match that for comment
 * updated some code to disambiguate ishidden where necessary
OPTION_MODERATE_PLEDGES redirect and creation

 * check moderation status when viewing a pledge
     - NB: we might not even need to update search.php, as pledges
       don't appear until they have signatories, and unmoderated pledges
       can't have those?
 * set pledge to hidden by default if option is set.
 * show a holding text to user (to be discussed with client)
    - NB: do we need a Next Action here instead?
TEMPORARY NOTES (rebase out)
example update
Allow remote "subadmin" access

This adds several new config variables:
    OPTION_ADMIN_REMOTE_SUBADMIN - whether to enable this
    OPTION_ADMIN_LOCKED_FIELDS_SUBADMIN - locked down subadmin access fields

These all use the CGI variable REDIRECT_REMOTE_USER, as per proxied access,
this should perhaps be made more general if will be deployed outside of
mySociety servers.

We also have a utility method for accessing a user object for these users,
get_admin_user.
Moderation details on moderating comment
default web-admin page to 'summary'
Moderation: Open/Closed admin => mode tabs

New $moderation_condition for searching for pledges to administer,
and refactored to mode tabs to make display more flexible.
CORRECT 'NOT' clause in moderation search
Basic moderation of pledges
moderation colour
Make /my page aware of moderation
tweak example config