Profile pictures have inconsistent alt text

[Jono M]

When editing profile pictures (Content -> Profile pictures), uploaded profile pictures have the filename as their alt text. This should be changed to the image title if provided.

The user's profile picture on the homepage has null alt-text - this should be changed to "Profile picture", the title of the picture, or something else more accessible.

Other places where profile pictures need alt text:
- Content -> Profile -> About Me
- Profile Info blocktype
- Group members list (on About and Members pages)
- List of forums and forum posts
- Admin -> Institutions -> Notifications (in table)
- Friends list
- My Friends blocktype (eg. on any profile page)
- Profile page of any user

Affects latest master

(WCAG Level A)