Add Message-ID to emails.

[donlorenzo]

RFC-2822 specifies that every email SHOULD have a Message-ID field. Some mail servers reject emails that do not have a Message-ID.
The eMails sent out by motioneye do not have a Message-ID.
This PR adds a Message-ID to the emails sent out by motioneye.

[zagrim]

Looks good to me. And a good addition, since being able to get email through seems to rely on a ton of things these days.

[zagrim]

Actually one thing that should change is the base branch of this PR, I think. The base should be dev branch of motioneye-project.

[zagrim]

I could also change the base branch of this PR but maybe it's better if you @donlorenzo check first that your change doesn't break if you rebase it on the current dev branch 😅 (there seem to be no changes to sendmail.py though between the branches)

[donlorenzo]

I withdraw this PR in favor of #3030 targeting the dev branch.

[MichaIng]

The base branch can be changed with the edit button at the top of the PR, and the a rebase onto the base branch can be done at the bottom of the page. No need to reopen a new PR.