Dashboard text blocks improvements

[bprusinowski]

As discussed with @bprusinowski after testing the feature:

• A bug when adding multiple charts from different datasets to the dashboards, they don't appear in the dashboards, only duplicated charts appear.
• Adding a possibility to remove a link from the text

• I would suggest to limit the options for choosing text styles to H1, H2, H3 and paragraph. Only to make it a bit more simple. Markdown of course could give more options if the user realize that they can use markdown, but at least it is not as obvious in the UI.
• It would be great to close the dialog when clicking outside if not too challenging to implement. Like this we would avoid having 3 action buttons on the bottom (save, remove link, cancel). Cancel button would not be necessary in this case.

Originally posted by @KerstinFaye in #1866