Adds modified date to snippets#show #1767

What's this MR do?

• Adds last modified date to snippets#show as suggested in #1767 (closed)
• Changes created_at to modified_at in snippets#index.
• I think this makes more sense for the users but I would understand if you would rather keep both as created_at
• Fixes padding on .btn-sm in master
• When implementing the new date I noticed the button formatting was a bit broken and noticed that there was a specificity war with the new btn-default mixin and the bootstrap .btn-sm selector.

Screenshots

snippets#show before

snippets#show after

admin/projects#index before

admin/projects#index after