Add method to copy secret values into the clipboard

[alexbrand]

When viewing a secret, it would be useful to have a way to copy the base64 decoded value of a secret into the clipboard.

Maybe something like this:

[rororoross]

I'd recommend using Clarity's copy-to-clipboard icon here

[tylerphelan]

we'd like this as well!

[sampeinado]

build service folks would appreciate this - especially if the button can give a bit of feedback to the user after its pressed! like a little "copied!" toast pops up for a second

ref:tdc ref:tbs

[GuessWhoSamFoo]

Adding this to 0.20 as it seems fairly trivial

[pivotal-mikegresham]

ref: tdc
ref: tbs