Customizable not secure DM warning and remove element.io advertisement

[poVoq]

Requirements

• This is a feature request and not a bug report. Otherwise, please create a new bug report instead.
• Please check to see if this request (or a similar one) already exists.
• It's a single feature. Please don't request multiple features in one issue.

Describe the feature you'd like

Its not very nice that the DM warning message currently advertises a commercial service (element.io).

It would be better if it was possible to customize this message, for example if the Lemmy admin also provides their own Matrix or XMPP server in combination with Lemmy.

In general it would be better if the default warning would only mention protocols (Matrix and XMPP at least) and not link to a specific (commercial) offer.

Thanks.