refactor: Extract narrow_banner module.

This is a mostly verbatim extraction.

I re-phrased one line of code to work around a lint
false alarm. (Look for `preamble` in the diff.)

There are about 8 lines missing coverage here, so
the new module might be a good candidate to get
100% line coverage on.

Before this change, you would need to remove 74
edges from our dependency graph to make it
acyclic. Now it's 72.

tools/linter_lib/custom_check.py

@@ -132,12 +132,6 @@ js_rules = RuleList(
         {
             "pattern": r"i18n\.t\(.+\).*\+",
             "description": "Do not concatenate i18n strings",
-            "exclude_line": {
-                (
-                    "static/js/narrow.js",
-                    'i18n.t("Some common words were excluded from your search.") +',
-                ),
-            },
         },
         {"pattern": r"\+.*i18n\.t\(.+\)", "description": "Do not concatenate i18n strings"},
         {