Save embed_html field as unicode

[idgserpro]

The embed_html field is of type Text:

sc.embedder/src/sc/embedder/content/embedder.py

Line 118 in 73fedda

embed_html = schema.Text(

Text fields are saved as unicode by default. However, the sc.embedder form treats the data before saving and saves it as a string:

sc.embedder/src/sc/embedder/content/embedder.py

Line 320 in 73fedda

data['embed_html'] = sanitize_iframe_tag(html.tostring(el))

This is bad because it causes incompatibility with data saved programmatically, with the transmogrifier, which saves the data as unicode.