Bug 1720348 gives MsgHeadersToolbar a CSS grid display, but only with a single-column layout.

We should convert MsgHeadersToolbar to:

• Use HTML, including its descendants where possible.
• No longer have it act as a customizable toolbar (see https://phabricator.services.mozilla.com/D141387#inline-780136).
• Take advantage of the grid display to align the remove buttons, field names, and inputs. This will allow us to remove the fixed width that we ask translators to set https://searchfox.org/comm-central/source/mail/locales/en-US/chrome/messenger/messengercompose/messengercompose.dtd#224

We can also better handle long pills whose max-content is longer than the row.

Converting this into a meta bug because it requires much more work.
Some of the initial steps to make this happen are:

• Use native HTML elements instead of HBOX and VBOX
• Convert the XUL label into HTML label
• Remove the separation between From, Recipients, and Subject areas in order to have a single GRID container
• Remove the single recipients containers (To, Cc, Bcc, etc) in order to have single elements styled by the same grid