share: Support choosing an account, if there are multiple

[rajveermalviya]

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Tests are TODO and will happen as part of #1787 in a follow-up PR.

Fixes: #1779

[chrisbobbe]

Thanks for building this!

I think my ideal version of this would have a slightly different UX:

• We don't add a new page, but instead show a modified version of the original page:
  • It has a nullable "currently selected account" state, controlled by a "Choose account" button somewhere like in the app bar
    • The button shows which account is selected, if any, making it persistently visible on the page (this should be helpful for accessibility; relying less on the user's memory/attention)
    • When pressed, the button opens a dialog with a list of accounts to select from
    • Maybe selection initializes to the last-visited account (if any), or maybe the dialog opens automatically at the beginning of the interaction
  • The page shows per-account data for the currently selected account (via PerAccountStoreWidget), or placeholder text if no account is selected.

I've pushed a rough draft of an approach like this, but it has some gaps:

• Track each realm's name and icon in database #1036
• Storing the realm logo in PerAccountStore
• Some UI strings to finalize
• maybe some visual design adjustments

If this approach sounds good, would you PTAL? Several of the commits are just cherry picks from #1802 which is close to merge.

[chrisbobbe]

Pushed with a rebase past #1802.