We don't actually use the reminder_type for bookmarks anywhere;
we are just storing it. It has no bearing on the UI. It used
to be relevant with the at_desktop bookmark reminders (see
fa572d3a7a6a3e107fdbc5f00e04d2bb990ab77d)
This commit marks the column as readonly, ignores it, and removes
the index, and it will be dropped in a later PR. Some plugins
are relying on reminder_type partially so some stubs have been
left in place to avoid errors.
This new column will be used to indicate that a bookmark
is at the topic level. The first post of a topic can be
bookmarked twice after this change -- with for_topic set
to true and with for_topic set to false.
A later PR will use this column for logic to bookmark the
topic, and then topic-level bookmark links will take you
to the last unread post in the topic.
See also 22208836c5f2adcf8c85a62d27b67968743b20f0
