Add not null constraint on User#preferred_language

What does this MR do?

Users created before 9.2 can have the preferred_language empty (NULL). This way, on the User preferences screen, an user might update the their language to Bulgarian, the first language of the language selector. To avoid that, and possible other problems, the preferred_language is marked as a not null and the users with empty preferred_language will be updated to the system default language (I18n.default_locale).

• 6d5364cf User#preferred_language default value was introduced in 9.2
• Related to: #221037 (closed)
• Issue to add the constraint validation: #229504 (closed)

Screenshots

$ bundle exec rails db:migrate
== 20200716093825 AddUsersDefaultValueOnPreferredLanguage: migrating ==========
-- change_column_default(:users, :preferred_language, {:from=>nil, :to=>:en})
   -> 0.0074s
== 20200716093825 AddUsersDefaultValueOnPreferredLanguage: migrated (0.0075s) =

== 20200716094825 AddNotNullConstraintToUsersPreferredLanguage: migrating =====
-- transaction_open?()
   -> 0.0000s
-- execute("ALTER TABLE users\nADD CONSTRAINT check_b8a6498b68\nCHECK ( preferred_language IS NOT NULL )\nNOT VALID;\n")
   -> 0.0037s
== 20200716094825 AddNotNullConstraintToUsersPreferredLanguage: migrated (0.0198s)

== 20200716095811 BackfillUserPreferredLanguage: migrating ====================
== 20200716095811 BackfillUserPreferredLanguage: migrated (0.0747s) ===========

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

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• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
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