properly handle instances that have empty settings (sourcegraph#4157)

In older (2.x) versions of Sourcegraph it was possible to save an empty string
as your user/org/global settings. This lead to various issues including just
completely breaking the settings page: https://github.com/sourcegraph/sourcegraph/issues/1645

We later fixed that issue and made it such that saving empty settings is
forbidden, but it doesn't appear we ever corrected such invalid settings. In
one customer environment we've encountered a user who is reporting their
settings are empty so we should handle this.

This is a quick fix which is cheap for us to maintain, so I opted for this
approach for now. The more correct (but also more complex) approach would be to
write a migration which transforms any empty settings into non-empty ones. I
don't think it is worth the time it would take right now.

log error (sourcegraph#4167)

Fix small editor (sourcegraph#3811)

* Fix small editor

Fixes sourcegraph#3645

* Add CHANGELOG entry

fix sourcegraph#4074

schema: Remove exclude.pattern from gitolite

This was accidently copied from the Bitbucket Server schema.

Fixes sourcegraph#4109

Added trailing commas for root level items in external services (sour…

…cegraph#4099)

Fixes sourcegraph#4078 and sourcegraph#4087