Several issues regarding Weight in Issuable sidebar

When the Issuable sidebar is collapsed, there are several issues regarding weight/milestone:

• The word "Weight" appears in the sidebar when only its icon should appear
• When no weight is set, the word "None" appears when we should only display "No"
• When weight is set, the weight number should appear under the icon instead of next to it

/cc @jschatz1

No weight set

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Weight set