Prepare for status update on filtering by church/country - solution proposal + estimate

[jkbcz]

1. WP - Add advanced group creator/selector
2. WP - Add group member table (automatically synced)
3. WP - Migrate from existing solution
4. Groups API - Add support for composite groups (based on other static or dynamic groups)
5. Groups API - notify about changes to dynamic/composite groups

[jkbcz]

@rvanoord my estimate for this is about 2 weeks:

1. WP - Add advanced group creator/selector - 2d
2. WP - Add group member table (automatically synced) - 2d
3. WP - Migrate from existing solution - 1d
4. Groups API - Add support for composite groups (based on other static or dynamic groups) - 2d
5. Groups API - notify about changes to dynamic/composite groups - 2d