update styling across app and create custom Button

[janethuangg]

this PR is largely just visual changes to clean up some of our page layouts and unify our button styling with a custom Button component

• Button turns blue on hover
• use linkTo prop if you want to link to a new page and onClick prop if you just want to set some onClick behavior (e.g. opening a modal
• if you want an icon, pass in an <Icon> component to the icon prop

[janethuangg]

@zhangrmatthew you can use the <Button> component in the login/signup pages once merged, along with the existing <TextInput> component

[janethuangg]

@cindykimxp @jharsch2401 you can also use the <Button> component on your respective pages!