Manually add necessary extension to postgres

Mind that I had to manually change the `db/schema.rb` file, as the
migrations for this application are broken: in 2017 we did a lot of
manual migrations that doesn't exist upstream (in decidim/decidim) and
that generates a problem locally and in testing where we can't run the
`db:migrate` task in a new database, but we need to do the
`db:schema:load` or `db:test:prepare` that loads the schema from the
schema.rb file instead of running each of the migrations.

Some examples of these files are:

- db/migrate/20170606113833_add_index_to_accountability_results_on_external_id.decidim_accountability.rb
- db/migrate/20170128100053_allow_erased_users.rb
- db/migrate/20170405084212_fix_email_uniqueness_index.rb
- db/migrate/20170125130555_remove_attachments_table.rb

Author: andreslucena

db/schema.rb

@@ -14,6 +14,7 @@
   # These are extensions that must be enabled in order to support this database
   enable_extension "ltree"
   enable_extension "pg_trgm"
+  enable_extension "pgcrypto"
   enable_extension "plpgsql"
 
   create_table "active_storage_attachments", force: :cascade do |t|