Make default avatars clickable

[dbkr]

BaseAvatar can be passed an onClick which is called when it's, er,
clicked, except when it was the default avatar in which case it
merrily ignored it. Make it not ignore it.

[richvdh]

er, let's not C&P this code.

either:

• always use AccessibleButton even when onClick is null (we could make it not do the tabIndex stuff in that case?)
• factor the common children out to a variable/funciton

[dbkr]

fair enough, ptal

[richvdh]

Sorry. I think we expect multiline JSX to be wrapped in (), as per the returns. Makes it rather more readable...

[dbkr]

done, ptaal