TECH-599: do not sync until employer record known to exist

[strdavis]

This pull request resolves an issue where when the user logs in for the first time, the sync operation can occur before the employer record exists, causing the sync to fail with code 403. The sync operation is now delayed until the employer record is created or known to exist. This pull request also fixes an issue where on the list screen a delay was sometimes visible before the loading indicator activated. Changes are as follows:

• [x] Implement EmployerContext to share some employer state between UserProfileModal and List components without passing state through many intermediate components.
• [x] When employer profile is created or known to exist, set flag in EmployerContext.
• [x] In List components, do not perform sync until employer record is known to exist.
• [x] In List components, render loading indicator immediately, so no delay is visible before it activates.

[sonarcloud[bot]]

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