Improve index tables

Stretch inputs to column width in place of fixed sizing.
Set minimum column widths for columns with form inputs.
Simplify table operations by removing `thead` and adding sentinel.
Manage empty table display with CSS only.

app/views/measurements/_form.html.erb

@@ -19,7 +19,8 @@
   <details id="quantity_select" class="center hexpand" open
            onkeydown="detailsProcessKey(event)">
     <summary autofocus>
-      <!-- TODO: Set content with CSS when span empty to avoid duplication -->
+      <%# TODO: Set content with CSS when span empty to avoid duplication; or
+        change to disabled input with prompt %>
       <span data-prompt="<%= t('.select_quantity') %>">
         <%= t('.select_quantity') %>
       </span>