Enable individual upload of avatar as e.g. `png` or `jpg`

Besides the gravatar.com integration, it would be great to allow individual upload of a picture to be set as avatar (similar to e.g. github or gitlab user profile settings)

1. navigate to `../accounts/profile/#profile` of your weblate instance
1. In section *Your avatar* provide a *file dialogue* to upload an avatar from your local disk
- provide and enforce size limits (e.g. pixel size or in kB)
1. Store uploaded picture in database (not in local file system as e.g. media)

Fallback: in case there is no individual picture uploaded, still use gravatar.com as fall back.

In relation to #2964 this would also help to identify a desired user when mentioning in comment function.