Login and logout UI

[GAsplund]

Key features:

• Example environment variables for next-auth
• Client-side login and logout buttons
• ActionButton now exposes the onClicked event
• Display a profile picture when logged in (with fallback image)
• Drop-down functionality separated into its own component
• Drop-down on profile picture to access account-related items
• Header turned back into server-side (to allow for SSR profile picture)
• Header CSR functionality moved into sub-components
• Closes #18

Known issues:

• Profile picture has no width (nor placeholder) during load, shifting components around

[GAsplund]

Everything seams to work as expected. I like that the "Min Profil" links to the gamma profile edit page! The one thing I was thinking was that maybe we want to have a tool tip or something similar that displays the logged in users nick when hovering the profile picture? Or maybe having the nick in the drop down? I don't see this as a must for this PR so if we want to do this later I am ready to approve in it's current state.

Great points! Having discussed different design choices with @Tussies, we agreed that the nick should probably be displayed somewhere else on the site (e.g. putting a greeting somewhere) and keep behavior as it exists in this branch. The main reason for this is that including the nick in the header introduces somewhat of a visual clutter as it is already relatively constrained vertically. Having the nick in the dropdown, while sounding good on paper, looks a little off and isn't super clear for the user what it does.