This adds consistency: The mention links with `@` was implemented
that way already. Instead of parsing the URL, we add some redundancy and
add another attribute: data-hashtag-id
So, what characters are valid for html attributes?
Read: https://stackoverflow.com/questions/925994/what-characters-are-allowed-in-an-html-attribute-name
Thanks to @Tirokk, who added some validations on the hahstag ids, I
think we are all set. If you try to write a hashtag with a `"` double
quotation mark for example, it gets automatically replaced with a valid
hashtag. If someone wants to send us invalid hashtag ids to the backend
directly, the regex there would filter it out.
