(WIP) Replace br with p and li in emails

[ruslandoga]

This makes the email templates more readable (I think) as they get formatted more predictably.

And this is also how GitHub renders HTML emails for comments / commits / etc.: <p> and <a> tags.

GitHub comment example

Content-Type: text/html; charset=UTF-8
Content-Transfer-Encoding: 7bit

<p></p>
<p dir="auto">Nice, I think we should give it a try!</p>

<p style="font-size:small;-webkit-text-size-adjust:none;color:#666;">&mdash;<br />Reply to this email directly, <a href="https://github.com/plausible/analytics/pull/4691#issuecomment-2418631060">view it on GitHub</a>, or <a href="https://github.com/notifications/unsubscribe-auth/AQFABUENVWQYD4WNC52KOWDZ35J6FAVCNFSM6AAAAABQBDLVB2VHI2DSMVQWIX3LMV43OSLTON2WKQ3PNVWWK3TUHMZDIMJYGYZTCMBWGA">unsubscribe</a>.<br />You are receiving this because you authored the thread.<img src="https://github.com/notifications/beacon/AQFABUDM5TZZ5LME5ZS4JYTZ35J6FA5CNFSM6AAAAABQBDLVB2WGG33NNVSW45C7OR4XAZNMJFZXG5LFINXW23LFNZ2KUY3PNVWWK3TUL5UWJTUQFFQZI.gif" height="1" width="1" alt="" /><span style="color: transparent; font-size: 0; display: none; visibility: hidden; overflow: hidden; opacity: 0; width: 0; height: 0; max-width: 0; max-height: 0; mso-hide: all">Message ID: <span>&lt;plausible/analytics/pull/4691/c2418631060</span><span>@</span><span>github</span><span>.</span><span>com&gt;</span></span></p>
<script type="application/ld+json">[
{
"@context": "http://schema.org",
"@type": "EmailMessage",
"potentialAction": {
"@type": "ViewAction",
"target": "https://github.com/plausible/analytics/pull/4691#issuecomment-2418631060",
"url": "https://github.com/plausible/analytics/pull/4691#issuecomment-2418631060",
"name": "View Pull Request"
},
"description": "View this Pull Request on GitHub",
"publisher": {
"@type": "Organization",
"name": "GitHub",
"url": "https://github.com"
}
}
]</script>

[aerosol]

Nice, I think we should give it a try!