File Preview - UI clean up

[mheppler]

There was some UI clean up that was missing in #3758. After reviewing with @TaniaSchlatter the to-do list is as follows:

• "Previews" btn text label should be "Preview Tools"
• White btn background for "access" is not appropriate on the Preview btn, it should be default gray
• Dropdown menu is aligned right, when it should be aligned left
• "Open {Preview Tool Name}" btn is small, not styled with Bootstrap, and breaks to a second line
• "Open {Preview Tool Name}" btn text label should be "Explore on {Preview Tool Name}", and needs an explore icon
• Preview tool iframe border needs styling, responsive Bootstrap classes

[mheppler]

Done and done.