Added a template for database changes

This template includes a rather large (but fairly easy to follow)
checklist. This checklist should hopefully take away quite a bit of work
from database specialists by requiring authors to provide all the
necessary info.

[ci skip]

.gitlab/merge_request_templates/Database Changes.md

+Remove this section and replace it with a description of your MR. Also follow the
+checklist below and check off any tasks that are done. If a certain task can not
+be done you should explain so in the MR body. You are free to remove any
+sections that do not apply to your MR.
+
+When gathering statistics (e.g. the output of `EXPLAIN ANALYZE`) you should make
+sure your database has enough data. Having around 10 000 rows in the tables
+being queries should provide a reasonable estimate of how a query will behave.
+Also make sure that PostgreSQL uses the following settings:
+
+* `random_page_cost`: `1`
+* `work_mem`: `16MB`
+* `maintenance_work_mem`: at least `64MB`
+* `shared_buffers`: at least `256MB`
+
+If you have access to GitLab.com's staging environment you should also run your
+measurements there, and include the results in this MR.
+
+## Database Checklist
+
+When adding migrations:
+
+- [ ] Updated `db/schema.rb`
+- [ ] Added a `down` method so the migration can be reverted
+- [ ] Added the output of the migration(s) to the MR body
+- [ ] Added the execution time of the migration(s) to the MR body
+- [ ] Added tests for the migration in `spec/migrations` if necessary (e.g. when
+  migrating data)
+- [ ] Made sure the migration won't interfere with a running GitLab cluster,
+  for example by disabling transactions for long running migrations
+
+When adding or modifying queries to improve performance:
+
+- [ ] Included the raw SQL queries of the relevant queries
+- [ ] Included the output of `EXPLAIN ANALYZE` and execution timings of the
+  relevant queries
+- [ ] Added tests for the relevant changes
+
+When adding indexes:
+
+- [ ] Described the need for these indexes in the MR body
+- [ ] Made sure existing indexes can not be reused instead
+
+When adding foreign keys to existing tables:
+
+- [ ] Included a migration to remove orphaned rows in the source table
+- [ ] Removed any instances of `dependent: ...` that may no longer be necessary
+
+When adding tables:
+
+- [ ] Ordered columns based on their type sizes in descending order
+- [ ] Added foreign keys if necessary
+- [ ] Added indexes if necessary
+
+When removing columns, tables, indexes or other structures:
+
+- [ ] Removed these in a post-deployment migration
+- [ ] Made sure the application no longer uses (or ignores) these structures
+
+## General Checklist
+
+- [ ] [Changelog entry](https://docs.gitlab.com/ce/development/changelog.html) added, if necessary
+- [ ] [Documentation created/updated](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/development/doc_styleguide.md)
+- [ ] API support added
+- [ ] Tests added for this feature/bug
+- Review
+  - [ ] Has been reviewed by UX
+  - [ ] Has been reviewed by Frontend
+  - [ ] Has been reviewed by Backend
+  - [ ] Has been reviewed by Database
+- [ ] Conform by the [merge request performance guides](http://docs.gitlab.com/ce/development/merge_request_performance_guidelines.html)
+- [ ] Conform by the [style guides](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md#style-guides)
+- [ ] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)