4.1: Display external images in PDF

[modelrailroader]

Currently, external images are not displayed in pdfs. Probably, this is because of a missing functionality in TCPDF. Maybe we can think about adding a function that replaces external images-urls from allowed media hosts with the base64-string.

We should think about this for 4.1

[modelrailroader]

You can assign me for this if you want to.

[modelrailroader]

@thorsten Please remove the 4.0.6 milestone as it is not absolutely necessary in 4.0. Maybe the 4.1 milestone is better in this case?