This commit migrates the `left_sidebar.html` Django template
to handlebars by creating a new file as `left_sidebar.hbs`
which is then rendered using `ui_init` module.
These are the minor changes introduced by virtue of template
migration -
- The `compute_show_invites_and_add_streams` function now
only concerns with the invite_to_realm_policy.
- Renamed the `compute_show_invites_and_add_streams` function
to `compute_show_invites` due to the above change.
- Fixes relevant `test_home.py` tests due to the above
changes.
Fixes part of #18792.
aryanshridhar