sage.databases: Use sage.feature to define database file system location

[mkoeppe]

... as done in #25825 for sage.databases.cremona and in #30607 for sage.databases.jones.

This would eliminate direct use of SAGE_SHARE (SAGE_LOCAL) from sage.databases.

CC: @antonio-rojas @kiwifb @tobiasdiez @videlec

Component: refactoring

Keywords: sd111

Issue created by migration from https://trac.sagemath.org/ticket/30819

[mkoeppe]

Changed keywords from none to sd111

[mkoeppe]

comment:3

Hoping we can make progress on this ticket this week - https://wiki.sagemath.org/days111

[mkoeppe]

comment:4

Sage development has entered the release candidate phase for 9.3. Setting a new milestone for this ticket based on a cursory review of ticket status, priority, and last modification date.