Handle invalid telegram html entities better

[jmbrunskill]

Describe the bug

{\"ok\":false,\"error_code\":400,\"description\":\"Bad Request: can't parse entities: Unsupported start tag \\\"h3\\\" at byte offset 0\"}"

https://core.telegram.org/api/entities#allowed-entities

Allowed entities
For example the following HTML/Markdown aliases for message entities can be used:

[messageEntityBold](https://core.telegram.org/constructor/messageEntityBold) => <b>bold</b>, <strong>bold</strong>, **bold**
[messageEntityItalic](https://core.telegram.org/constructor/messageEntityItalic) => <i>italic</i>, <em>italic</em> *italic*
[messageEntityCode](https://core.telegram.org/constructor/messageEntityCode) => <code>code</code>, `code`
[messageEntityStrike](https://core.telegram.org/constructor/messageEntityStrike) => <s>strike</s>, <strike>strike</strike>, <del>strike</del>, ~~strike~~
[messageEntityUnderline](https://core.telegram.org/constructor/messageEntityUnderline) => <u>underline</u>
[messageEntityPre](https://core.telegram.org/constructor/messageEntityPre) => <pre language="c++">code</pre>,

To Reproduce

Steps to reproduce the behaviour:

1. Create a notification with an h3 tag or p (looks like most things aren't supported!)
2. Send notification - causes telegram error

Expected behaviour

Not entirely sure?? Would be good to show what the problem is better...

We probably will need different telegram and email templates eventually?
Maybe we can convert html to markdown first then send it as markdown to telegram?

Additional context

[jmbrunskill]

Should we just support markdown in notify?

https://crates.io/crates/pulldown-cmark