also:
* fix select chevrons appearing above modals
* add custom errors and use them for http errors, also now appear above
modals.
* functional logout button
* slightly cleaned up admin.ts
the DOMInvite class represents an invite on the dom, and modifying its
attributes applies the changes on the web page. Email send status
message is now on the right of the invite and represented by an icon.
Hovering reveals the "Sent to"/"Failed to send to" message.
the backend code has been added back in, and the build process has been
modified so that no output files are left outside of build/. Admin page
loads and logs in correctly, nothing else functionality-wise yet.