[Repo] Add possibility to create user choice list

[szyb]

Synopackage Proxy Repository is now fixed to only 2 channels: All & SynopackageChoice. User can choose just from those two.

This task is to add possibility to the user choose their own source servers. This leads to generate unique (per set of servers) URL that will contain encoded sources

Backend:
We can use https://github.com/ullmark/hashids.net to encode & decode a set of numbers which will corresponds to chosen sources.

Frontend:
Add page to generate URL:

• Choose servers (with "Generate" button)
• Save generated URLs by the user in the local storage and visualize generated URL with sources
• Add page that can decode sources to check which of those are included (for a specific version given in User Settings)