Add avatar to issue and MR pages header

closes #14395
# For Issues

### Before
![Screen_Shot_2016-03-18_at_4.27.25_PM](/uploads/b097c8e304c9737e2f46ee25667829bf/Screen_Shot_2016-03-18_at_4.27.25_PM.png)

### After
![Screen_Shot_2016-03-18_at_4.22.10_PM](/uploads/f7d2b12c9455950c2e660b33e27c1827/Screen_Shot_2016-03-18_at_4.22.10_PM.png)

# For Merge Request

### Before
![Screen_Shot_2016-03-18_at_4.27.09_PM](/uploads/50c7b6ee346f749f448647004ae6031b/Screen_Shot_2016-03-18_at_4.27.09_PM.png)

### After
![Screen_Shot_2016-03-18_at_4.24.17_PM](/uploads/abd37ccf030e0b6d10521a6937206de6/Screen_Shot_2016-03-18_at_4.24.17_PM.png)

See merge request !3301