[ticket/358] Remove use of T_EXT_THEME_PATH in attachments side

Additionally, we'll now also test that the portal doesn't show errors
while using subsilver.

B3P-358
[ticket/358] Remove uses of T_EXT_THEME_PATH in prosilver files

B3P-358
[ticket/358] Remove uses of T_EXT_THEME_PATH in subsilver

B3P-358
[ticket/358] Remove T_EXT_THEME_PATH from overal_header_head_append

Also, the responsive CSS file for the portal has been renamed to
portal_responsive.css in order to prevent conflicts with other extensions.

B3P-358
Merge pull request #362 from marc1706/ticket/358

[ticket/358] Remove use of T_EXT_THEME_PATH