fix: Chosen perk item buttons now reflect chosen status

Before, a chosen perk item's button would read "Choose this gift" even if you had chosen it, which was confusing. This MR updates the display of a chosen perk's button such that it reads "Your gift".

Additional button elements were added to donate.html.jinja with custom class attributes, and static/sass/sections/_forms.scss has some new SASS in it which adds rules for when to display each button type. It should work like so at this point:

• Perks with a greater donation minimum than the current donation amount will read "Donate $XXX instead"
• Perks available to select will read "Choose this gift"
• The selected perk will read "Your gift"

Closes #61 (closed).