We render navbar entries in two places: the topbar (just under the

"Review Board" label) and the mobile menu. In the mobile menu, the

navbar entries should be rendered using the sidebar layout/classes,

rather than the topbar ones. Unfortunately, extension-provided navbar

entries would only render using the topbar layout, which made them look

very ugly in the mobile menu.
This change adds an optional argument to the templatetag we use to

render the navbar items, which lets us choose a template to use.

Ran unit tests.
Looked and saw that the "Reports" entry added by Power Pack no longer

  looked terrible in the mobile menu.