More Precise Proposal Pricing

Two updates shipped for proposals: you can now set markup on individual line items instead of one blanket number for the whole proposal, and you can mark specific line items or pricebook items as non-taxable so they're excluded from the tax calculation automatically. Together, these give you a lot more control when a proposal mixes labor, materials, and tax-exempt items, without fudging the math or working around it outside CompanyCam.