Add umap embed support

Closes #19

• Add embed support for Umaps: You can now embed a custom map created at https://umap.openstreetmap.de right into our website

How to embed a map

Create a umap and copy the URL to embed it (e.g. https://umap.openstreetmap.de/de/map/coli-karte_18214?scaleControl=false&miniMap=false&scrollWheelZoom=false&zoomControl=true&allowEdit=false&moreControl=true&searchControl=null&tilelayersControl=null&embedControl=null&datalayersControl=true&onLoadPanel=none&captionBar=false).

In the article, you now have two options:

1. Add an "Embed" block and paste the URL there
2. Insert the embed shortcode in a paragraph, by simply interrupting your normal text with [embed]https://umap.openstreetmap.de/...[/embed]. This method can also be used to specify some additional parameters, especially width and height (see https://codex.wordpress.org/Embed_Shortcode for more details)