MedieInfo U2.7 Slim test

Please compile before uploading and note:
- Program bytes
- Maximum bytes
- RAM bytes

Functional test:
1. SD card and existing folders
2. JPG display and upload
3. Slide editing modal
4. MQTT plain payload
5. MQTT JSON field
6. Image, slide and mixed playlists
7. OTA firmware page

Only the two test/demo generators were removed.
