comments: reviews_base.html template

* Moves reviews.html to reviews_base.html; broken move commit.

Signed-off-by: Adrian-Tudor Panescu <adrian.tudor.panescu@cern.ch>
comments: reviews.html template

* Re-adds reviews.html template extending from reviews_base.html; fixes broken
move commit.

Signed-off-by: Adrian-Tudor Panescu <adrian.tudor.panescu@cern.ch>
previews: move to previewer

* Renames the previews module to previewer; broken move commit.

Signed-off-by: Adrian-Tudor Panescu <adrian.tudor.panescu@cern.ch>
previewer: post-move fixes

* Fixes issues generated by module renaming.

Signed-off-by: Adrian-Tudor Panescu <adrian.tudor.panescu@cern.ch>
previewer: refactoring

* Changes preview view function arguments, passes request arguments.

* Sends only file reference from view to plugins.

* Fixes PEP8.

* Adds previewer preference order, settable via CFG_PREVIEW_PREFERENCE, which
is a dictionary such as {'.pdf' : ['pdftk', 'gview']}, specifying that 'pdftk'
it the first choice for PDFs.

Signed-off-by: Adrian-Tudor Panescu <adrian.tudor.panescu@cern.ch>
utils: unicode fix

* Accepts unicode as well as strings for escape_shell_arg().

Signed-off-by: Adrian-Tudor Panescu <adrian.tudor.panescu@cern.ch>