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

[chrisbobbe]

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.

[chrisbobbe]

Milestoning post-launch because the legacy apps don't offer this.

[AshutoshKhadse23]

Hello,
I would like to work on it

issue.1215.mp4

[chrisbobbe]

Sure, I've just assigned you @AshutoshKhadse23; please send a PR when your work is ready for review. Since this issue has the "post-launch" milestone, the core team won't have much time to help in the near term. As always, please self-review your work to check that it's clear and correct.