Select all threads

[jbpenrath]

Bug Report

Problematic behavior
As a user when I select all threads in the thread list, I expect all existing threads are selected not only the subset loaded in the list.

Expected behavior/code
Endpoint to modify threads and their flags should allow to update all threads at once.

[sylvinus]

Gmail UI is good for this. First selects all visible, then gives the option to select all with count