Migrate initial global header fetch away from accounts

After #5829 , we will no longer be using React's Context API for quartz /accounts data. Once that is the case, global header should be modified as follows:

- Initially store only the user's current account/org based on data from /identity. Do not store full list of accounts until menu is expanded.
- Change conditional for whether to render switch accounts button from checking for the _length_ of the /accounts array to checking the _value_ of the account/org count properties from the user object.
- Fetch /accounts _only_ once global header is expanded.
- Discuss with design appropriate behavior when header is in a 'loading' state of fetching (or failing to fetch) from /accounts.

Start with the global header and the #5829 Context API removal, then trace its current /identity and /accounts data flow. Done means the header initially uses only the current account/org, fetches /accounts on expansion, checks count properties for the switch button, and has an agreed loading or failure behavior with design.

Written by the indexing model from the issue text.