Refactor form input styles

* Fix issue where button is rendered taller in flexbox, due to default
  margin on input elements
* Refactor/improve all login and signup views

app/views/invitations/index.html.erb

@@ -6,14 +6,13 @@
       <p class="mb-8">
         Invite your friends to a Kosmos account by sharing an invitation URL with them:
       </p>
-      <ul>
+      <ul class="md:w-3/4">
         <% @invitations_unused.each do |invitation| %>
-          <li class="font-mono mb-1 flex gap-1 md:block"
-              data-controller="clipboard">
-            <input type="text" disabled class="md:w-3/4 flex-1"
+          <li class="font-mono mb-2 flex gap-1" data-controller="clipboard">
+            <input type="text" disabled class="relative grow"
                    value="<%= invitation_url(invitation.token) %>"
                    data-clipboard-target="source" />
-            <button id="copy-user-address" class="btn-md btn-icon btn-blue flex-none w-auto"
+            <button id="copy-user-address" class="btn-md btn-icon btn-blue shrink-0 w-auto"
                     data-clipboard-target="trigger" data-action="clipboard#copy"
                     title="Copy to clipboard">
               <span class="content-initial">