Remove timestamps and duplicate url from templates

[fmunshi]

Summary

• Removes timestamps from posts since the mattermost app already shows a timestamp when message is posted.
• Since the time shown by the github plugin is always UTC it is confusing since it is in a different timezone
• The consensus from this issue here Make the timezone configurable #109 seemed to be to remove the timestamp entirely

Fixes #109

Screenshots

Heres what it currently looks like

Heres what the proposed changes look like

[levb]

OT: 0/5 @aaronrothschild @jasonblais with the date stamp removed, should the link and subject be consolidated into a single line, to save space? (or another way to save a line?)

[marianunez]

Thank you for your contribution! 🎉

[DHaussermann]

Tested and passed.
Ensured that the timestamp is no longer included for delivery of this event.
No issues found
LGTM!

[mattermod]

This issue has been automatically labelled "stale" because it hasn't had recent activity.
A core team member will check in on the status of the PR to help with questions.
Thank you for your contribution!

/cc @jasonblais @hanzei

[jasonblais]

We could go as far as combine the first three lines into one via

#new-pull-request [pull-request-title](pull-request-link) by @pull-request-author

I am completely 0/5 on what's optimal, though. Worth a separate UX design ticket?

cc @aaronrothschild quick reminder with PM review :)

[aaronrothschild]

LGTM