Add saved replies to DB

What does this MR do and why?

This MR adds necessary database changes to support storing and managing Saved Replies for single user.

Migration

$ env VERBOSE=true bundle exec rake db:migrate:down VERSION=20220216110023
== 20220216110023 CreateSavedReplies: reverting ===============================
-- drop_table(:saved_replies, {:if_exists=>true})
   -> 0.0141s
== 20220216110023 CreateSavedReplies: reverted (0.0142s) ======================

$ env VERBOSE=true bundle exec rake db:migrate:up VERSION=20220216110023
== 20220216110023 CreateSavedReplies: migrating ===============================
-- create_table(:saved_replies, {})
-- quote_column_name(:name)
   -> 0.0000s
-- quote_column_name(:content)
   -> 0.0000s
   -> 0.0221s
== 20220216110023 CreateSavedReplies: migrated (0.0222s) ======================

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 #352938 (closed)