Allow locking branch in protected namespaces

[isc-pbarton]

If a namespace is a "protected namespace" in the HCC recommended source control workflow, it should always be tracking one branch. But it's easy to forget which namespace you're in and use the source control menus to switch branch.

Idea: add a new namespace-level setting "lock branch". If this is enabled, the New Branch and Switch Branch options in the source control menu should be disabled.