Make lockIsStale() protected [#3586194]

Problem/Motivation

ContentLock::locking is ~100 lines of code, "hardcoding" some UI/UX patterns. It also calls lockIsStale() that is defined as private. I'd suggest changing it to protected so inherited classes could easily rewrite the locking() eg. if needs to change the UI/UX.

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes

Issue fork content_lock-3586194

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3586194-make-lockisstale-protected changes, plain diff MR !104
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