Allow updates and enhanced handling of users for the Issue Field User step in approval definitions.

Scope of changes:

1. New options in the Issue Field User step form:

   • Treat users as a group – a group step will be created with all users retrieved from the selected issue field.

   • Dynamically update users when the step is active – users will be added or removed based on the current contents of the issue field.

2. When the "Dynamically update users" option is selected:

   • Users will receive notifications when:

     • they are added to the step – informing them they need to cast a vote,

     • they are removed from the step – informing them they no longer need to vote or their decision is not valid any more.

3. If "Treat users as a group" is selected, the following fields (same as in Group step) should be available:

   • Number of votes required for group step to pass

   • Exclude users from group

   • Exclude approval creator from group

   • Skip the group from the approval if it doesn't contain any users

From the technical side, we need to listen for changes in issues and check whether the change affects any custom fields of type user or the description field.
If so, we should find approvals that have an active Group step created based on a Field User step with the options "Treat users as a group" and "Dynamically update users" enabled, and update the list of assigned users accordingly.