Remove the dependency on the velocity template library, and all code
related to reading and parsing velocity templates.
With this change, Gerrit no longer recognizes or handles Velocity
templates (*.vm) for emails. Site administrators must replace any
Velocity templates with the equivilant Soy templates before upgrading
to a Gerrit version that includes this change.
Update the mail config documentation to clarify that Velocity is no
longer supported.
Change-Id: I9942eb1a94adfa677c5a25634f3a1fee68c680fa
cb6597828b