Add drop for type and removed if not exists

- added drop of the type
- removed if not exists on table creation
- added a better explaination as to why the reverse ordering is needed

Author: eleijonmarck

@app/db/migrations/current.sql

@@ -6,10 +6,30 @@ example of a table that could be created after setup complete
 - uncomment the statements below
 - save the file and graphile-migrate will update the schema into database automagically
 - any "drop" statements should be at the top (in reverse order)
+reverse order as we could have references
+from the second created table to the first created table
+
+here is an pseudo example:
+DROP C;
+DROP B;
+DROP A;
+CREATE A;
+CREATE B;
+CREATE C;
+
+if references exists from B -> A
+we have to DROP B before DROP A we have references that point to A from B
+
+In the example below post and feed need to be
+deleted in reverse order due to feed -> post
 */
 
--- drop table if exists feed;
--- drop table if exists post;
+-- UNCOMMENT FROM HERE --
+
+-- drop table if exists app_public.feed;
+-- drop table if exists app_public.post;
+
+-- drop type if exists app_public.post_topic;
 
 -- create type app_public.post_topic as enum (
 --   'discussion',
@@ -18,7 +38,7 @@ example of a table that could be created after setup complete
 --   'showcase'
 -- );
 
--- create table if not exists app_public.post (
+-- create table app_public.post (
 --   id               serial primary key,
 --   author_id        integer not null references app_public.users(id),
 --   headline         text not null check (char_length(headline) < 280),
@@ -35,7 +55,7 @@ example of a table that could be created after setup complete
 -- comment on column app_public.post.body is 'the main body text of our post.';
 -- comment on column app_public.post.created_at is 'the time this post was created.';
 
--- create table if not exists app_public.feed (
+-- create table app_public.feed (
 --   id               serial primary key,
 --   posts            integer not null references app_public.post(id),
 --   created_at       timestamp default now()