database: Call a specification 'jobset' instead of 'project'.

This removes the possibility to filter specifications by branch, because
branches were previously called 'jobset'.  But it doesn't matter because later
on, specifications will have as many branches as inputs.  And people should
filter by specification name instead.

* doc/cuirass.texi (Build Information, Latest builds): Remove 'jobset',
replace 'project' with 'jobset'.
* src/cuirass/http.scm (build->hydra-build): Idem.
* tests/database.scm (db-get-builds): Idem.
* tests/http.scm (build-query-result, /api/latestbuilds?nr=1&jobset=guix,
/api/latestbuilds?nr=1&jobset=gnu): Idem.
* src/cuirass/database.scm (db-format-build, db-get-builds): Don't associate
builds with branches (which were called 'jobset' afterwards).
(db-get-builds): Remove the #:project filter.