Bug 5506 Shorten .po file comment lines

Bug 5506 Shorten .po file comment lines

In opac/staff templates .po files, we have comment lines contextualizing
extracted text in templates. Path to template are absolute. For example, we can
have:

  #: /home/katrin/kohaclone/koha-tmpl/intranet-tmpl/prog/en/...
     modules/cataloguing/addbiblio.tmpl:585

The first part of the pathname is useless. With this patch, we just keep
relative path to tempalte from Koha template main directory. The above example
becomes:

  #: intranet-tmpl/prog/en/modules/cataloguing/addbiblio.tmpl:585

To be applied on [3.2]

Signed-off-by: Chris Cormack <chrisc@catalyst.net.nz>