Add button to copy testcase as base64 text

Currently there are two options: download and view.

I mostly develop via SSH, so there is no convenient way to put testcase to my working environment:

1. download testcase
2. in terminal `cd` to directory with tescase
3. `base64 -i` testcase
4. copy text
5. on remote `echo "PASTE" | base64 -d > tescase`

Alternative is to use`scp`, but that is really more typing.

Steps 1-4 could be replaced with a button that copies base64 encoded testcase into clip-buffer.

Even better, if testcase is copied alongside with echo, base64 and test case name, so it will be just paste-and-enter.

Review the implementation behind the existing testcase Download and View options, then trace where the testcase data reaches the browser. The feature should copy base64 text to the clipboard, ideally as a paste-and-run command including echo, base64 decoding, and the testcase name; verify the result from the testcase view.

Written by the indexing model from the issue text.