Make Initial Sync timeout value configurable

[pano9000]

Hi,

this is a "reminder" issue, so that this does not get buried/fogotten.
Originally discussed here: #5370

Idea is to have the Initial Sync value configurable by some means, and also set it a higher default value.

[pano9000]

Setting the initial value via ENV is fixed with the PR TriliumNext/Notes#1036, but it might be worth to make this setting available right in the Setup UI – I'll interpret this Issue as the latter now.

The higher value default is still open, but to be discussed:
Currently we are at 120_000 ms which is 120 sec / 2 mins – shall we just go with 10min straight away?