Fix unexpected return type of User::idFromName()

Fix unexpected return type of User::idFromName()

The user_id is an unsigned integer in the database. But not all database
abstractions we use are guaranteed to return integer values as PHP
integers. Sometimes it's a string and needs an integer cast first.

Want proof? Search for usages of this method. Almost all add an (int)
cast. This is weird and should not be necessary.

Change-Id: If1d706f73350fca5b3a0f1e0de59e4518162445b