There are two separate ways this is tracked:

1. Session data (see userAlreadySentThanksForId in ApiFlowThank).
2. Cookie (flow-thanked).

If we really want to refactor this and do it 100% properly, these are both wrong. Session data and cookie data both expire. We need something persistent on the server like a DB table; then the server can render it correctly (per this bug) (side note, server already tries to do this right for ordinary revision thanks, but it uses the session which will eventually expire).

The client doesn't even really need to cache it then, AFAICT: After AJAX, just update the DOM to de-link and say 'Thanked', then on next page reload, the server will render it.

This is a regression from rETHAccdca0d0d7e0: Support updated flow frontend.

Steps to reproduce the issue:

1. On any Flow board click to 'Thank' anyone.

2. 'Thank' changes to 'Thanked' and a proper notification is sent.

3. Click to change the sorting order of a board - 'Thanked' changes to 'Thank again'.

4. Click that reverted 'Thank' - the following page will be displayed:

JS in the Console:

[FLOW] Failed to getInstanceByElement: no $container.length

[FLOW] Error in component handler: makeContentInteractive TypeError: component.emitWithReturn is not a function

5. 'Send thanks' can be clicked again - no new notifications will be sent which is good.
6. Stay on the Special page-Send thanks page and log out.
7. Log in as a user to whom Thank was sent - the user upon login will be on the page.
8. Click on 'Send thanks' - 'Thank action failed. Please try again.'