Add support for btree_gist postgres extension

btree_gist is now required by gitlab as of gitlab!32335 (merged)

Log from Local Installation Attempt in Vagrant

gitlab: ---- Begin output of "bash"  "/tmp/chef-script20200527-9366-1y8eg0u" ----
gitlab: STDOUT: psql:/opt/gitlab/embedded/service/gitlab-rails/db/structure.sql:3: NOTICE:  extension "plpgsql" already exists, skipping
gitlab: psql:/opt/gitlab/embedded/service/gitlab-rails/db/structure.sql:5: ERROR:  permission denied to create extension "btree_gist"
gitlab: HINT:  Must be superuser to create this extension.
gitlab: rake aborted!
gitlab: failed to execute:
gitlab: psql -v ON_ERROR_STOP=1 -q -X -f /opt/gitlab/embedded/service/gitlab-rails/db/structure.sql gitlabhq_production
gitlab: 
gitlab: Please check the output above for any errors and make sure that `psql` is installed in your PATH and has proper permissions.
gitlab: 
gitlab: /opt/gitlab/embedded/service/gitlab-rails/lib/tasks/gitlab/db.rake:53:in `block (3 levels) in <top (required)>'
gitlab: /opt/gitlab/embedded/bin/bundle:23:in `load'
gitlab: /opt/gitlab/embedded/bin/bundle:23:in `<main>'
gitlab: Tasks: TOP => db:structure:load
gitlab: (See full trace by running task with --trace)
gitlab: STDERR: 
gitlab: ---- End output of "bash"  "/tmp/chef-script20200527-9366-1y8eg0u" ----