BibSched: improve pager and editor conf

• Improves pager configuration by changing default pager set in CFG_BIBSCHED_LOG_PAGER to /usr/bin/less.

• Introduces new editor configuration variable CFG_BIBSCHED_EDITOR with default value of /usr/bin/vim.

• For both editor and pager, use respectively the EDITOR and PAGER environment variables. (closes #987)

Co-authored-by: Samuele Kaplun <samuele.kaplun@cern.ch>