backups: add more info, keep 1 of each version opt

backup's filename format has changed, and includes the commit too now. a
Backup struct for going to/from the filename has been added, and the
option "keep 1 backup from each version" has been added, leaving the
most recent backup from each version always. All pre-this-commit backups
are considered the same "old" version.

html/admin.html

@@ -372,6 +372,7 @@
                             <tr>
                                 <th>{{ .strings.name }}</th>
                                 <th>{{ .strings.date }}</th>
+                                <th>{{ .strings.version }}</th>
                                 <th class="table-inline justify-center">{{ .strings.backupDownloadRestore }}</th>
                             </tr>
                         </thead>