Research and validation of Postgres 10

We plan to bundle and support PG 10 soon: #3308 (closed). We need to do some work so we are well prepared to implement

Goals for this issue are:

1. Does GitLab with work PG 10, open any issues if not
2. Add to CI now, a PG 10 set of jobs to ensure we don't break PG10 support. Note that this should also the end-to-end gitlab-qa tests as well.
3. What would it take to bundle PG 10
4. Define the migration process. (For example, Optional until GitLab 13, then required.)