Add import/export feature for bookmarks (#14956)

* Add ability to export bookmarks

* Add support for importing bookmarks

* Add bookmark import tests

* Add bookmarks export test

config/locales/en.yml

@@ -842,6 +842,7 @@ en:
       request: Request your archive
       size: Size
     blocks: You block
+    bookmarks: Bookmarks
     csv: CSV
     domain_blocks: Domain blocks
     lists: Lists
@@ -918,6 +919,7 @@ en:
     success: Your data was successfully uploaded and will now be processed in due time
     types:
       blocking: Blocking list
+      bookmarks: Bookmarks
       domain_blocking: Domain blocking list
       following: Following list
       muting: Muting list