linter: Make duplicate html tag id detection work with archives.

We modify check-templates to check for duplicate id's in archive
templates and app templates separately. This means we are allowing
app and archive templates to potentially use same id's. This is
needed because we intend to re use some js from the main app and
having same id's help achieve that.
Note: We haven't up until this point actually added archive
templates. This commit is more of a preparatory commit for merging
the basic archive infra.

tools/check-templates

@@ -49,6 +49,7 @@ def check_html_templates(templates, all_dups):
     if 'templates/zerver/team.html' in templates:
         templates.remove('templates/zerver/team.html')
 
+    def check_for_duplicate_ids(templates: List[str]) -> Dict[str, List[str]]:
         template_id_dict = build_id_dict(templates)
         # TODO: Clean up these cases of duplicate ids in the code
         IGNORE_IDS = [
@@ -84,8 +85,20 @@ def check_html_templates(templates, all_dups):
                           "' present at following files:")
             for fn in fns:
                 print(fn)
+        return bad_ids_dict
 
-    if list(bad_ids_dict.keys()):
+    bad_ids_list = []  # type: List[str]
+    archive_templates = list(filter(
+        lambda fn: ('templates/zerver/archive' in fn),
+        templates))
+    templates = list(filter(
+        lambda fn: ('templates/zerver/archive' not in fn),
+        templates))
+
+    bad_ids_list += list(check_for_duplicate_ids(archive_templates).keys())
+    bad_ids_list += list(check_for_duplicate_ids(templates).keys())
+
+    if bad_ids_list:
         print('Exiting--please clean up all duplicates before running this again.')
         sys.exit(1)