Adds an editable text label to the gr-change-metadata for modifying the
topic of a change. Introduces the gr-editable-label as a UI element for
editable text labels. Editing the topic is only allowed if the user is
logged in and has the topic-edit permission.
This is a redo of change 78110, which had been rebased wrong and had to
be reverted. https://gerrit-review.googlesource.com/c/78110/
Bug: Issue 4102
Change-Id: Ia7bb7b803a9fdd15a5f9f0f8eecf77bdd3ee4ba7
f1d7d87d6f