Use Gravatar profile pictures if no picture is set on the Requirements Bazaar or if it does not exist

[BenediktHensen]

Is your feature request related to a problem? Please describe.
Since most profile pictures on Requirements Bazaar are currently broken, it might be a good idea to directly query Gravatar for the user's profile picture so that we do not just get default avatar images everywhere.

Describe the solution you'd like
When querying the profile picture, there should be a fallback order for the displayed image:

1. Check if the picture provided by the Requirements Bazaar is set and whether it exists.
2. Check Gravatar based on the mail address of the profile and get the profile picture from there.
3. Show the default avatar picture that VIAProMa has if none of the above solutions yielded a picture.

Additional context
To get a Gravatar image, we need to convert the mail address to a hash using MD5. Possible pre-processing of the mail address is described here: https://de.gravatar.com/site/implement/hash/

After that, the picture can be requested, as described here: https://de.gravatar.com/site/implement/images/