Allow MarkDown and HTML in Annoucements

[jacobwod]

Annoucements (added in #161) is a long-standing experimental feature.

The current implementation is limited as it only allows pure text. No MD nor HTML is allowed:

While allowing MD would increase the weight of the component significantly, I'll wait with that. But adding support for pure HTML formatting shouldn't affect performance.

[jacobwod]

On a second thought: I'm allowing HTML too. This is needed if we want to make line breaks (using <br> or <p>). This wouldn't be possible in the current solution because a carrige return isn't allowed inside a JSON string:

{
      "id": 7,
      "active": true,
      // Not possible:
      "text": "Some text with a line break at the end.
                   A new line here.

                   We can do a paragraph too.",
      "showOnlyOnce": false,
      "type": "info"
    }

However, using HTML, we can do this:

{
      "id": 7,
      "active": true,
      "text": "Some text with a line break at the end<br>A new line here.<br><br>We can do a paragraph too.",
      "showOnlyOnce": false,
      "type": "info"
    }

The result will look like the second (blue) box:

[jacobwod]

OK, pretty much done. This should significantly broaden the limits of what's possible with our Announcements function in Hajk.

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