Storage limit: 80% warning and limit in the web interface

- after each successful backup the stored size is recorded; at 80% of a
  location's limit administrators get a notification (once a day)
- locations list shows stored size and limit; the add form has a
  'maximum size' field that explains that empty means unlimited and
  billed by the provider; PUT /api/targets/{id}/limit (password
  confirmation)

Tested on NC34 with the OVH test bucket: a backup above the limit stops,
a successful one at 81% sends the warning.

appinfo/info.xml

@@ -5,7 +5,7 @@
     <name>NG Backup</name>
     <summary>Encrypted, incremental backups of your Nextcloud with restore from the web interface</summary>
     <description><![CDATA[Work in progress, not ready for use.]]></description>
-    <version>0.0.17</version>
+    <version>0.0.18</version>
     <licence>agpl</licence>
     <author>Dillard Blom</author>
     <namespace>NgBackup</namespace>