Move "Create new snapshots" button into header

In my previous commit, I moved the accept/reject buttons on the snapshot
page into the header. This commit does a similar thing for the "Create
new snapshots button" on the URL page, which brings a nice bit of
consistency to the app.

Change-Id: Ic5da615a2d7b5a8cf1a7188bbbf3552264731ee2
Use btn-primary instead of btn-info

The button on the ProjectsController#index page uses btn-primary, and I
think it makes sense for the other similar buttons to be the same. This
commit makes that happen.

Change-Id: I0f1a4660d4f86695751cb165b0acabd73b2f3b6f
Put "Start new sweep" "button" in a paragraph

This gives it a little space following the element, which looks nicer.

Change-Id: Ic9ab4d433f21058a31262449086a3eca4825794b