Make it slightly easier for extensions to hook into page protection.

Make it slightly easier for extensions to hook into page protection.

Split parts of doUpdateRestrictions() into several more concise functions.
The biggest drawback is increasing $dbw->encodeExpiry() calls times 3. This is
not much of a problem given that's a very inexpensive function and it is only
called very few times ($limit holds 2 values in current codebase)

Added $reason to the ProtectionForm::save hook, so that other code hooking into
and adding their own settings into page protection form can also process the
reason for the protection change.

Change-Id: I879290ed83e4e47e9561d4c352fbd50c07d7e18a