Add experiments and experiment_users tables

What does this MR do?

This MR adds 2 tables for tracking which users are enrolled for which experiments conducted by the Growth team. This makes for easier analysis when we want to measure eg. stage adoption for users who have been subjected to a particular experiment.

In addition, in the current MR an event for the onboarding_issues experiment is recorded when the experiment is enabled and a user selects continue on this page https://gitlab.com/users/sign_up/welcome.

The following tables are added:

experiments
id
name

Migration output

== 20200731085019 CreateExperiment: migrating =================================
-- table_exists?(:experiments)
   -> 0.0004s
-- create_table(:experiments)
   -> 0.0138s
-- transaction_open?()
   -> 0.0000s
-- execute("ALTER TABLE experiments\nADD CONSTRAINT check_e2dda25ed0\nCHECK ( char_length(name) <= 255 )\nNOT VALID;\n")
   -> 0.0008s
-- execute("ALTER TABLE experiments VALIDATE CONSTRAINT check_e2dda25ed0;")
   -> 0.0005s
== 20200731085019 CreateExperiment: migrated (0.0185s) ========================

experiment_users
id
experiment_id
user_id
group
created_at
updated_at

Migration output

== 20200731090553 CreateExperimentUser: migrating =============================
-- create_table(:experiment_users)
   -> 0.0029s
-- add_index(:experiment_users, :experiment_id)
   -> 0.0018s
-- add_index(:experiment_users, :user_id)
   -> 0.0017s
== 20200731090553 CreateExperimentUser: migrated (0.0065s) ====================
== 20200731201408 AddForeignKeyToExperimentOnExperimentUsers: migrating =======
-- add_foreign_key(:experiment_users, :experiments, {:column=>:experiment_id, :on_delete=>:cascade})
-> 0.0030s
== 20200731201408 AddForeignKeyToExperimentOnExperimentUsers: migrated (0.0044s)
== 20200731201834 AddForeignKeyToUserOnExperimentUsers: migrating =============
-- add_foreign_key(:experiment_users, :users, {:column=>:user_id, :on_delete=>:cascade})
-> 0.0117s
== 20200731201834 AddForeignKeyToUserOnExperimentUsers: migrated (0.0131s) ====

Issue: #230469 (closed) and #229057 (closed)