Update Review Board how-to

[danielledeleo]

Issue #80 revealed that the Review Board docs have changed. We should verify that our how-to still works as written and update the broken links.

# from 'make check'
howto/reviewboard.rst:51: [https://www.reviewboard.org/docs/manual/dev/admin/installation/creating-sites/#changing-permissions] (Anchor 'changing-permissions' not found)
howto/reviewboard.rst:1: [https://www.reviewboard.org/docs/manual/dev/admin/installation/linux/#before-you-begin] (Anchor 'before-you-begin' not found)
howto/reviewboard.rst:23: [https://www.reviewboard.org/docs/manual/dev/admin/installation/linux/#installing-review-board] (Anchor 'installing-review-board' not found)