Add confirmation modal to "Sync now" LDAP button

📖 What does this MR do?

Related to #330902 (closed)

Adds a confirmation modal to the "Sync now" LDAP button to warn users that synchronizing LDAP removes direct members' access.

💻 Local testing

1. Setup LDAP using the GDK LDAP guide
2. Login as LDAP user john with password password
3. Create a group
4. Navigate to Group -> Settings -> LDAP Synchronization
5. Choose LDAP Group cn for Sync method
6. Choose group1 from the LDAP Group cn dropdown
7. Click Add synchronization
8. Navigate to Overview -> Members
9. Click Sync now button

📹 Videos

Before	After
Screen_Recording_2021-06-22_at_8.07.11_AM	Screen_Recording_2021-06-22_at_8.05.11_AM

🚦 Does this MR meet the acceptance criteria?

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