Updates application `MainLayout`

[allanlasser]

I updated the application's main layout in response to lessons learned:

• Both sidebars are now optional slots (also mobile buttons are not rendered if not provided).
• Added a global header with user and org data, with a placeholder tag for holding breadcrumb or other navigation elements.
• The slot names haven't changed, so pages that already use MainLayout should not need to be changed at all.

I also made some data fetching changes:

• I renamed lib/api/users.js to lib/api/accounts.ts and renamed the test file accordingly.
• I added a method for fetching an organization given an ID.
• I fetch the user's organization and add it to app context at the same time as the user.

I also fixed some styles.

(I'm not satisfied with the mobile header as-is, but the point in this PR is that the slotted sidebars are visible as drawers from there. If we want to move around user and org menus, we can do that later on.)

Screenshots

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[eyeseast]

Getting an overflow somewhere. Might be in the top nav. This is on my iPhone XR.