compose: Insert a <time> element ("global times")

From beta feedback:

There doesn't seem to be a way to enter dates/times in the new beta app. I would love to have an option in the toolbar to insert a date, and have that bring up a date picker.

This issue is for offering the "global times" feature as part of the message-composing UI.

We already parse and render these elements in messages (that was #354; we still have #922 open for an uncommon edge case). We should offer a date-picker UI that helps the user insert the Markdown for one of these in the compose box.

This will be another button in the row beneath the content input; see Figma:

The icon should come from the Figma icons page.

Implementation

Probably should start by trying out showDatePicker.