rework backup root

notes:
* backup root cannot come from backend. for dynamic mounts backend cannot know where it is mounted
* backupConfig is 3 parts - format / mount / password . there is also this rootPath (which should not be in db)
* password should be stored separately in settings at some point
* format has to be passed along everywhere because we allow restore from  same backupConfig but different format. we do this by saving the format in the backups table

fixes #819

src/routes/test/backups-test.js

@@ -228,7 +228,7 @@ describe('Backups API', function () {
             tmp.backupFolder = BACKUP_FOLDER;
             tmp.limits = { copyConcurrency: 34 };
 
-            const response = await superagent.post(`${serverUrl}/api/v1/backups/config`)
+            const response = await superagent.post(`${serverUrl}/api/v1/backups/config/storage`)
                 .query({ access_token: owner.token })
                 .send(tmp);
 
@@ -247,7 +247,7 @@ describe('Backups API', function () {
 
     describe('create', function () {
         before(async function () {
-            await backups.setConfig({
+            await backups.setStorage({
                 provider: 'filesystem',
                 backupFolder: '/tmp/backups',
                 format: 'tgz',