Bug 32422: Hardcoded paths in _common.scss precent using external node_modules

Bug 32422: Hardcoded paths in _common.scss precent using external node_modules

This patch makes:

- gulpfile.js add the parent dir's node_modules to the sass include path
- removes hardcoded paths from _common.scss

To test:
1. Have the latest KTD image (this is not KTD specific, but easier to
   test)
2. Make sure you don't have any node_modules/ dir in your koha clone
3. Run:
   $ kshell
  k$ yarn build --view opac
=> FAIL: You get:
kohadev-koha@kohadevbox:/kohadevbox/koha$ yarn build --view opac
yarn run v1.22.19
$ gulp build --view opac
[18:27:38] Using gulpfile /kohadevbox/koha/gulpfile.js
[18:27:38] Starting 'build'...
Error in plugin 'sass'
Message:
    koha-tmpl/opac-tmpl/bootstrap/css/src/_common.scss
Error: File to import not found or unreadable: ../../../../../node_modules/bootstrap/scss/functions.
        on line 37 of koha-tmpl/opac-tmpl/bootstrap/css/src/_common.scss
        from line 1 of koha-tmpl/opac-tmpl/bootstrap/css/src/opac.scss
>> @import "../../../../../node_modules/bootstrap/scss/functions";

   ^

4. Apply this patch
5. Repeat 3
=> SUCCESS: It builds!
6. Sign off :-D

Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>