Configuration scheme context screen: unfriendly applicable context management

Summary

When a user wants to add or select more projects on a "Configuration scheme context" screen a "control" key usage is required to avoid current configuration reset (already selected projects). In case of more than hundred projects on a list, any mistake makes that more painful.
From a user perspective, there's a big risk of unintentional mistake and configuration data loss.

Steps to reproduce

How to get to the screen:

• Navigate to the Custom Fields page, locate the desired custom field and choose:
  • cog icon > Configure
  • locate desired context > Configure
    • go to page section: Choose the applicable context

The selection process of context (projects) is nasty, a horrible user experience. See screenshot below.

Expected results

The more user-friendly solution for a project list that may contain> 500 entries.

User comments:

I would like to have separate list projects with already subscribed to this setting of options and with "delete" button near every project name.
Plus separate list of all projects with button "add" or "append" which would add project to the previous list. There could be many potential solutions for this problem. We have 500 projects now, it's really hard to manage scheme/context.
Or at least change this old multi-select style (ctrl button required) to modern multi-select with autocomplete feature, something like for fields: Labels, Fix Versions etc.