Saved searches should not be subject to 50 repositories limit

Created by: slimsag

Today, saved searches are subject to a 50 repository limit in the same way that users are: https://github.com/sourcegraph/sourcegraph/issues/6826

Because saved searches aggressively scope queries to a smaller subset using before: and after: time scoping filters, they should not be subject to this limit as they were before.

Proposed (somewhat non-ideal) implementation: If before: or after: are in use, do not show the alert. When the alert is shown, advise scoping to fewer repositories OR using before: / after: (users must do this to create the saved search).