Billing fixes (tips disappear; statistics bounds)

Tip disappears in custom period tab.
Statistics for custom period include both bounds.

Issue: altai-57, altai-58
Change-Id: I5c140c7d1c55bac520308fdfee58e2aca4706935
Do not allow user to revoke his own admin permissions

Issue: altai-74
Change-Id: I6af8b8841d4d0d2995b035e86f49a7e1c41fa004