Improve weight icon

The scale as an icon for issue weight seems a little easier to recognize. I also removed the icon from the issuable form because none of the other fields have icons. We have similar icons for assignee and milestone, but they are not present here. If we want the weight icon, I propose we add the respective icons for each field.

Screenshots:

Issue list

Screen_Shot_2015-12-21_at_3.10.56_PM

Screen_Shot_2015-12-21_at_3.08.37_PM

Form

Screen_Shot_2015-12-21_at_3.12.15_PM

Screen_Shot_2015-12-21_at_3.07.19_PM

Sidebar

Screen_Shot_2015-12-21_at_3.13.01_PM

Screen_Shot_2015-12-21_at_3.13.15_PM

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