Add 'rule' to the search operators

In change I0602cbff, we added a 'rule' search operator to allow
searching for submit rules that evaluate to some status. See
documentation in [1]. In this change we add 'rule' to the search
operators to allow for auto-complete in the search bar.

[1]
https://gerrit-review.googlesource.com/Documentation/user-search.html#rule

Change-Id: I66adb77003f2ff92817bcc5d49767ea290555f0c
diff --git a/polygerrit-ui/app/elements/core/gr-search-bar/gr-search-bar.ts b/polygerrit-ui/app/elements/core/gr-search-bar/gr-search-bar.ts
index 7c65c3f..8257734 100644
--- a/polygerrit-ui/app/elements/core/gr-search-bar/gr-search-bar.ts
+++ b/polygerrit-ui/app/elements/core/gr-search-bar/gr-search-bar.ts
@@ -110,6 +110,7 @@
   'reviewer:',
   'reviewer:self',
   'reviewerin:',
+  'rule:',
   'size:',
   'star:',
   'status:',