Add repository_read_only column to NamespaceSettings table

What does this MR do?

In order to support the ability to change a Group repository's storage, we need to be able to mark a Group as having a read-only repository, in the same way it is done for Project repositories.

This MR adds the boolean column to the NamespaceSettings table as the first step in that process.

Epic &5166 (closed)

Refs #299052 (closed)

Migrate Up

== 20210122073805 AddRepositoryReadOnlyToNamespaceSettings: migrating =========
-- add_column(:namespace_settings, :repository_read_only, :boolean, {:default=>false, :null=>false})
   -> 0.0015s
== 20210122073805 AddRepositoryReadOnlyToNamespaceSettings: migrated (0.0070s)

Migrate Down

== 20210122073805 AddRepositoryReadOnlyToNamespaceSettings: reverting =========
-- remove_column(:namespace_settings, :repository_read_only)
   -> 0.0012s
== 20210122073805 AddRepositoryReadOnlyToNamespaceSettings: reverted (0.0076s)

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Related to #299052 (closed)