[web client] insert many files before current in playlist gets truncated with dynamic mode on

(Enhancment request)

Steps to reproduce:
1. Open the website client, enable dynamic mode, playback
2. Select a big (> 10) number of files from the library
3. Add these files _before_ the currently playing song

Experienced behaviour:

4.1. The files are added. Then all but the last 10 are deleted from the playlist

Expected behaviour:

4.2. The files are added, and stay: dynamic mode should take care of the "manually added" flag even for "already played" (or supposed to be) files