feat(base): Add snapper setup stack

Creates separate snapper services for root and home directories. Does
_not_ yet set up any btrfs mounts or ensure that they are mounted at the
locations.

books/templates/snapper-configurations/home.j2

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+# subvolume to snapshot
+SUBVOLUME="/home"
+
+# filesystem type
+FSTYPE="btrfs"
+
+# btrfs qgroup for space aware cleanup algorithms
+QGROUP=""
+
+# fraction or absolute size of the filesystems space the snapshots may use
+SPACE_LIMIT="0.5"
+
+# fraction or absolute size of the filesystems space that should be free
+FREE_LIMIT="0.2"
+
+# users and groups allowed to work with config
+ALLOW_USERS="{{ user_name }}"
+ALLOW_GROUPS=""
+
+# sync users and groups from ALLOW_USERS and ALLOW_GROUPS to .snapshots
+# directory
+SYNC_ACL="no"
+
+# start comparing pre- and post-snapshot in background after creating
+# post-snapshot
+BACKGROUND_COMPARISON="yes"
+
+# run daily number cleanup
+NUMBER_CLEANUP="yes"
+
+# limit for number cleanup
+NUMBER_MIN_AGE="1800"
+NUMBER_LIMIT="4"
+NUMBER_LIMIT_IMPORTANT="2"
+
+# create hourly snapshots
+TIMELINE_CREATE="yes"
+
+# cleanup hourly snapshots after some time
+TIMELINE_CLEANUP="yes"
+
+# limits for timeline cleanup
+TIMELINE_MIN_AGE="1800"
+TIMELINE_LIMIT_HOURLY="10"
+TIMELINE_LIMIT_DAILY="2"
+TIMELINE_LIMIT_WEEKLY="1"
+TIMELINE_LIMIT_MONTHLY="1"
+TIMELINE_LIMIT_QUARTERLY="0"
+TIMELINE_LIMIT_YEARLY="0"
+
+# cleanup empty pre-post-pairs
+EMPTY_PRE_POST_CLEANUP="yes"
+
+# limits for empty pre-post-pair cleanup
+EMPTY_PRE_POST_MIN_AGE="1800"

books/templates/snapper-configurations/root.j2

@@ -0,0 +1,55 @@
+# subvolume to snapshot
+SUBVOLUME="/"
+
+# filesystem type
+FSTYPE="btrfs"
+
+# btrfs qgroup for space aware cleanup algorithms
+QGROUP=""
+
+# fraction or absolute size of the filesystems space the snapshots may use
+SPACE_LIMIT="0.5"
+
+# fraction or absolute size of the filesystems space that should be free
+FREE_LIMIT="0.2"
+
+# users and groups allowed to work with config
+ALLOW_USERS="{{ user_name }}"
+ALLOW_GROUPS=""
+
+# sync users and groups from ALLOW_USERS and ALLOW_GROUPS to .snapshots
+# directory
+SYNC_ACL="no"
+
+# start comparing pre- and post-snapshot in background after creating
+# post-snapshot
+BACKGROUND_COMPARISON="yes"
+
+# run daily number cleanup
+NUMBER_CLEANUP="yes"
+
+# limit for number cleanup
+NUMBER_MIN_AGE="1800"
+NUMBER_LIMIT="6"
+NUMBER_LIMIT_IMPORTANT="4"
+
+# create hourly snapshots
+TIMELINE_CREATE="yes"
+
+# cleanup hourly snapshots after some time
+TIMELINE_CLEANUP="yes"
+
+# limits for timeline cleanup
+TIMELINE_MIN_AGE="1800"
+TIMELINE_LIMIT_HOURLY="6"
+TIMELINE_LIMIT_DAILY="5"
+TIMELINE_LIMIT_WEEKLY="2"
+TIMELINE_LIMIT_MONTHLY="1"
+TIMELINE_LIMIT_QUARTERLY="1"
+TIMELINE_LIMIT_YEARLY="0"
+
+# cleanup empty pre-post-pairs
+EMPTY_PRE_POST_CLEANUP="yes"
+
+# limits for empty pre-post-pair cleanup
+EMPTY_PRE_POST_MIN_AGE="1800"