Simplify pledging process

[CooperSmout]

Currently the user has to click two 'Pledge' buttons to make a pledge -- first on the campaign page, which opens up a pledge page with editable settings, and then again at the bottom of that page (after adjusting settings) to sign the actual pledge.

It would be better if all this could happen on the same page, i.e. using a single button placed below the campaign details (Rationale, Criteria, and any settings that are relevant to that campaign), which doubles as a login button if the user has not logged in already.

Note that this Issue is related to Issue #8, which asks for the settings to be made optional for each campaign.