Fix UI regression from moving the header button

Problem:

When merging in #214,
the Percy.io visual regression tests weren't run
because our quota for the month had been used up.

In that PR, we re-aligned the button in the header.
This change accidentally affected content below the header.

Solution:

Change the alignment from `baseline` to `flex-start`.
This keeps the button from affecting content below the header.

app/assets/stylesheets/administrate/components/_header.scss

@@ -1,5 +1,5 @@
 .header {
-  align-items: baseline;
+  align-items: flex-start;
   display: flex;
   justify-content: space-between;
   margin-bottom: $base-spacing * 2;