fix(6.x forms) editable grid buttons (#193)

- since 7.3 theme resources are accessed in a relative way using the URL
../theme. As a result, the path of the images declared in the CSS is
relative to the css directory in the theme so resources are not
retrieved from the war portal/ directory anymore.
We cannot remove them from there, though, because old forms look&feels
might still use them.
The solution is to just copy the two images we use inside the
appropriate directory in the theme.

Covers [BS-16006](https://bonitasoft.atlassian.net/browse/BS-16006)