Use the full width in the email header

I was initially foiled by MJML not supporting auto-growing columns
(or rather, them not being able to get that to reliably work in
Outlook Desktop:
mjmlio/mjml#1669 (comment)),
and thus set the right-hand column (the one containing the "Sign
in" link) to 60%, hoping that that would look reasonable enough
on most screen sizes — on large screens, it would be fairly right-
aligned, and on smaller screens, it wouldn't be pushed to the next
line.

However, the second thing that confused me was that MJML's columns
don't take effect on viewport widths smaller than 480px, i.e.
they're wrapped in media queries starting at that level.

If I just only test on wider viewports, then I can actually just
set the columns to together span 100%. The image itself still has
a fixed width of 200px, so that won't auto-grow anyway. Only
downside is that the "Sign in" link will still be pushed to the
next line on screens under 480px, but I guess that's just
something we'll have to live with.

src/emails/templates/EmailHeader.tsx

@@ -17,7 +17,12 @@ export const EmailHeader = (props: Props) => {
       padding="10px 20px"
       background-color="white"
     >
-      <mj-column width="200px" vertical-align="middle">
+      <mj-column
+        width="60%"
+        padding-left="0"
+        padding-right="0"
+        vertical-align="middle"
+      >
         <mj-image
           alt={l10n.getString("public-nav-name")}
           src={`${process.env.SERVER_URL}/images/email/monitor-logo-with-bg.png`}
@@ -26,7 +31,12 @@ export const EmailHeader = (props: Props) => {
           align="left"
         />
       </mj-column>
-      <mj-column width="60%" vertical-align="middle">
+      <mj-column
+        width="40%"
+        padding-left="0"
+        padding-right="0"
+        vertical-align="middle"
+      >
         <mj-text align="right">
           <a
             href={`${process.env.SERVER_URL}/user/dashboard/fixed?utm_source=monitor-product&utm_medium=email&utm_campaign=${props.utm_campaign}&utm_content=sign-in-us`}