fix(admin): sponsor optimization

Description

fix(admin): sponsor optimization

• Check if some sponsor exists before enabling buttons.
• Logic for obtaining sponsors was moved to dialogs.
• Show loading while obtaining sponsors.
• Work with User instead of RichUser.

How to test

• On the VO Sponsored members page, check if buttons are enabled (disabled if there are no sponsors for VO) and try to open dialogs for create/generate/copy/sponsor members.
• On the Member detail page, check the card for sponsoring the given member.
• Check both scenarios when you are SPONSOR and when you are PERUNADMIN.
• Testing VO with 130k sponsors and 10k sponsored members: https://gui-dev.perun-aai.org/organizations/4051

Author's checklist

• I have followed the contribution guidelines
• This MR has been tested or does not change functionality
• I have added relevant merge request dependencies (if this MR has any)
• I have added the correct labels
• I have assigned reviewers (if any are relevant)
• I have edited the documentation (if the changes require it) or I have noted the need for the change if I do not have access to the documentation
• I have marked all introduced BREAKING CHANGES or necessary DEPLOYMENT NOTES in the commit message(s)

Reviewer's checklist

• This MR has been tested or does not change functionality
• This MR has correct commit message format

Other information

Run E2E tests after BE MR is merged.

Related issues

Merge after: perun!4314 (merged)

re STR-1161