Create a template file for agenda emails

[gkatsev]

Thought this might be useful to have instead of hunting down old emails.

[nigelmegitt]

I usually just go to the previous agenda's GitHub issue, but happy to think about other ways to make this work easily.

Not sure if it's worth it but we could make this change to the agenda issue template at .github/ISSUE_TEMPLATE/agenda---ttwg-meeting-yyyy-mm-dd.md

Another thought, perhaps we could add a Chair's action list at the top, which would include stuff like:

* [ ] Fix agenda number
* [ ] Fix timeanddate date and time
* [ ] Fix ICS file href
* [ ] Check the correct agenda items are present
* [ ] Cross check against https://www.w3.org/PM/Groups/agenda.html?gid=34314 for GitHub issues labelled as "agenda".

[himorin]

how about to include above action list part and skelton-ish heading part of previous meeting issues?? Like

Agenda to follow

### Time and Joining Details: 16:00 - 17:00 (1 hours) UTC

Please be ready for a prompt start at 16:03am UTC time.

Our teleconference is scheduled with reference to UTC Time, the correct time of this teleconference in your locale may change.

Check https://www.timeanddate.com/worldclock/fixedtime.html?iso=202YMMDDT16

See W3C Calendar link for joining details: <Copy&Paste URL from w3.org/events/meetings/>

(if you can't see the joining details and wish to join please contact me)

#### Future meetings and recent activity:

* See the [TTWG project board](https://github.com/w3c/ttwg/projects/1) for upcoming meetings.
* See individual agenda items for recent activity on specific products.
* See the [TTWG home page](https://www.w3.org/AudioVideo/TT/#recent-activity) for links to repositories and recent activity.