Add rate limiting for user email lookup

What does this MR do and why?

Adds rate limiting option to user lookup by email. That way we can resist to user email probation

Migration output

== 20211213142344 AddSettingsUserEmailLookupLimit: reverting ==================
-- transaction_open?()
   -> 0.0001s
-- remove_column(:application_settings, :user_email_lookup_limit)
   -> 0.0325s
== 20211213142344 AddSettingsUserEmailLookupLimit: reverted (0.1734s) =========

== 20211213142344 AddSettingsUserEmailLookupLimit: migrating ==================
-- transaction_open?()
   -> 0.0001s
-- add_column(:application_settings, :user_email_lookup_limit, :integer, {:null=>false, :default=>10})
   -> 0.0365s
== 20211213142344 AddSettingsUserEmailLookupLimit: migrated (0.0701s) =========

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #346140