Block editing of recurring contribution total when more than one line item exists

The Recurring contribution UI should not allow people to edit the total amount for transactions with more than one line item as our current model has no way of recording the line items that should be used for subsequent transactions.

Implementation
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1. Preprocess in ./Contribute/Form/UpdateSubscription.php should check if there are > 1 line_item associated w/ this recurring contribution.
2. If TRUE, 'amount' field should be set to 'view only' (not editable).
3. Please remove the span class 'font-red' in the WARNING message (see below) since it makes the text unreadable.
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$userAlert = "<span class='font-red'>" . ts('Updates made using this form will change the recurring contribution information stored in your CiviCRM database, but will NOT be sent to the payment processor. You must enter the same changes using the payment processor web site.'
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NOTE: We have decided we should block these changes through the UI until it can be handled properly. We have agreed to put it into 4.4 & 4.6 - assuming it is small we might want to put it in 4.5 too for our own sanity.

NOTE: Edits will still be permitted for single-line-item contributions & the repeat.transaction api will be enhanced to support this per CRM-16397.