This commit deduplicates the code for `build_realm_logo_widget` and
`build_realm_night_logo_widget`. It deduplicates the common code for
`build_realm_day_mode_logo_widget` & `build_realm_night_mode_logo_widget`
into tthe function `build_realm_logo_widget`.
The night logo synchronization on the settings page was perfect, but
the actual display logic had a few problems:
* We were including the realm_logo in context_processors, even though
it is only used in home.py.
* We used different variable names for the templating in navbar.html
than anywhere else the codebase.
* The behavior that the night logo would default to the day logo if
only one was uploaded was not correctly implemented for the navbar
position, either in the synchronization for updates code or the
logic in the navbar.html templates.
This adds a new realm_logo field, which is a horizontal-format logo to
be displayed in the top-left corner of the webapp, and any other
places where we might want a wide-format branding of the organization.
Tweaked significantly by tabbott to rebase, fix styling, etc.
Fixing the styling of this feature's loading indicator caused me to
notice the loading indicator for the realm_icon feature was also ugly,
so I fixed that too.
Fixes#7995.