Merge branch 'improve_weight_icon' into 'master'

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](/uploads/1578b649a7481fcc99e098ccde59d846/Screen_Shot_2015-12-21_at_3.10.56_PM.png)

![Screen_Shot_2015-12-21_at_3.08.37_PM](/uploads/f58e87c615ff68d97b509f0974833619/Screen_Shot_2015-12-21_at_3.08.37_PM.png)

### Form

![Screen_Shot_2015-12-21_at_3.12.15_PM](/uploads/54338e7c65cc9ee2645f4c7a27a0be7b/Screen_Shot_2015-12-21_at_3.12.15_PM.png)

![Screen_Shot_2015-12-21_at_3.07.19_PM](/uploads/61761287d261dfa1b37d4a1f98a9a442/Screen_Shot_2015-12-21_at_3.07.19_PM.png)

### Sidebar

![Screen_Shot_2015-12-21_at_3.13.01_PM](/uploads/2e69888e7438946df45eb988e16c65f0/Screen_Shot_2015-12-21_at_3.13.01_PM.png)

![Screen_Shot_2015-12-21_at_3.13.15_PM](/uploads/78ffcbf426f71cddbff731dd04a48687/Screen_Shot_2015-12-21_at_3.13.15_PM.png)


See merge request !88

app/views/projects/issues/_issue.html.haml

@@ -47,7 +47,7 @@
     - if issue.weight
        
       %span
-        = icon('magnet')
+        = icon('balance-scale')
         = issue.weight
     .pull-right.issue-updated-at

app/views/shared/issuable/_sidebar.html.haml

@@ -63,7 +63,7 @@
               = link_to 'Edit', '#', class: 'edit-link'
         .value
           - if issuable.weight
-            = icon('magnet')
+            = icon('balance-scale')
             = issuable.weight
           - else
             .light None

doc/workflow/issue_weight.md

@@ -8,7 +8,7 @@ You can set the weight of an issue during its creation, by simply changing the
 value in the dropdown menu. You can set it to a numeric value from 1 to 9.
 This value will appear on the right sidebar of an individual issue, as well as
-in the issues page next to a distinctive magnet icon.
+in the issues page next to a distinctive balance scale icon.
 As an added bonus, you can see the total sum of all issues on the milestone page.