BibKnowledge: lxml port get_kbt_items_for_bibedit

* Replaces usage of `libxml2` and `libxslt` with `lxml` in
  `get_kbt_items_for_bibedit` function.

Signed-off-by: Jiri Kuncar <jiri.kuncar@cern.ch>
BibKnowledge: fix get_kbt_items_for_bibedit

* Uses either libxml2 or lxml depending on which library is present.
  (closes #1879) (closes #1895)

Signed-off-by: Yoan Blanc <yoan.blanc@cern.ch>
Tested-by: Tibor Simko <tibor.simko@cern.ch>
installation: lxml recommended

* Recommends lxml now that libxml2 and libxslt1 are fully optional.

Signed-off-by: Tibor Simko <tibor.simko@cern.ch>
MiscUtil: detect Apache version
Drone: add .drone.yml

* Adds `.drone.yml` for Drone continuous integration.

Signed-off-by: Federico Poli <federico.poli@cern.ch>
Co-authored-by: Alessio Deiana <alessio.deiana@cern.ch>
Merge pull request #76 from fpoli/prod-drone

Drone CI and Updating BibKnowledge to use lxml