Allow to customize Submit button label and tooltip

Ibe3547942 renamed "Submit" button to "Merge Change" with justification
why to prefer "Merge Change" label over "Submit".

I5d7e1d21a reverted that change and restored it back to "Submit" with
another justification for why "Merge Change" can be misleading.

This change externalize the label name and tooltip for Submit button
to enable site administrator to customize it.

Bug: issue 2667
Change-Id: Iec78b31051f6b0022af1f337f5d492b78c7df096
Merge "Allow to customize Submit button label and tooltip"