introduce serverpath macro

for now, computes path to backup directory, appending the relative path
of UploadBackupDir to the absolute one from the UploadURL

Also added example for store.php to hint

core/ui/ControlPanel/Saving.tid

@@ -10,10 +10,10 @@ http://$(userName)$.tiddlyspot.com/backup/
 <$reveal type="nomatch" state="$:/UploadName" text="">
 <$set name="userName" value={{$:/UploadName}}>
 <$reveal type="match" state="$:/UploadURL" text="">
-<a href=<<backupURL>>><$macrocall $name="backupURL" $type="text/plain" $output="text/plain"/></a>
+<<backupURL>>
 </$reveal>
 <$reveal type="nomatch" state="$:/UploadURL" text="">
-<<lingo TiddlySpot/RelativeURL>>
+<$macrocall $name=serverpath source={{$:/UploadURL}} target={{$:/UploadBackupDir}}>>
 </$reveal>
 </$set>
 </$reveal>
@@ -33,5 +33,4 @@ http://$(userName)$.tiddlyspot.com/backup/
 |<<lingo TiddlySpot/UploadDir>> |<$edit-text tiddler="$:/UploadDir" default="." tag="input"/> |
 |<<lingo TiddlySpot/BackupDir>> |<$edit-text tiddler="$:/UploadBackupDir" default="." tag="input"/> |
 
-<<lingo TiddlySpot/Hint>>
-
+<<lingo TiddlySpot/Hint>>