ui adjustments

Minor user interface adjustments.

localization: German

Pile is now available in German.

new cleanup mode

• new cleanup mode allows to remove multiple bookmarks in one fell swoop
• counter badge now shows the correct number when multiple bookmarks are removed or added
• less web-extension permissions are used
• pinned bookmarks are not remove anymore when using the context menu
• code re-work: await instead of raw Promise handling
• several css adjustments
• upgraded from gulp 3 to gulp 4

new icon, some minor cosmetic changes

* new icon. the old one was too generic and caused confusion for some

users.
* searchbar unfolding: cosmetic changes