Fix delete and switch to using must_be

This patch follows Merlijn van Deen's suggestion to use must_be
instead of calling APISite.login directly. It also fixes Page.delete
to support switching between bot and sysop account and consequently
partially fixes bug 60278.

Change-Id: Ibc94da881fce312f42ce58fd9a440d1fb5e06109
Merge "Fix delete and switch to using must_be"