Instant picture upload subfolder naming

### Actual behaviour
-Tell us what happens

Naming convention:
yyyy = Year in digit
mm = Month in digit
dd = Day of the month in digit
Condition:
Settings - Use subfolders - yes

With uploading pictures to the choosen upload folder a subfolder yyyy is generated and a subfolder thereof mm and the picture file is put into this folder. Which results in
InstantUpload/yyyy/mm/filename (= InstantUpload/2019/08/20180801-140303.jpg)
This does create multiple mm folders (same name) with different content in different years.

### Expected behaviour
-Tell us what should happen

The naming of the subfolder(s) should be configurable with a selectable given pattern.
Then according to the need of the user a pattern like
InstantUpload/yyyy/yyyymm/ (= InstantUpload/2018/201808/)
or
InstantUpload/yyyy/yyyymm/yyyymmdd/ (=InstantUpload/2018/201808/20180801/)
would be possible.

This would integrate with other picture management software.

### Steps to reproduce
1. Setting - subfolders - yes
2. Take a picture
3. Verify the folder structure in InstantUpload

### Environment data
Android version: 4.4.2

Device model: SM-G900FD

Stock or customized system: Samsung stock

Nextcloud app version: 1.4.3

Nextcloud server version: 12.0.1

### Logs
#### Web server error log
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#### Nextcloud log (data/nextcloud.log)
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