Remove the `DEFAULT_THREADS_SHOWN` limit and the View More / Collapse
row from the sidebar thread list. When a project group is expanded, all
of its threads are now shown. Fold/unfold of project group headers is
preserved.

### Changes

- Remove `ListEntry::ViewMore` variant and `render_view_more`
- Remove `DEFAULT_THREADS_SHOWN`, `group_extra_batches`,
`set_group_visible_thread_count`, `expand/collapse_thread_group`
- Remove `visible_thread_count` from `ProjectGroupState`,
`SerializedProjectGroupState`, `ProjectGroup`, and persistence
- Remove `ShowMoreThreads` / `ShowFewerThreads` actions and vim
keybindings

cc @danilo-leal

Release Notes:

- N/A